I’ve heard this term on Lemmy and pretty much nowhere else. What is a tankie?

Yes exactly. And that's why I don't jump to the assumption that Zelenskyy himself is corrupt or he is covering for the corruption of others.
A dude that would tolerate corruption doesn't say 'I need ammunition not a ride'. He could have bailed at any point, still could. But he shows no signs of losing resolve.
If Zelenskyy is trying to keep inner circle corruption private to avoid public spectacle, I suspect that whoever is found guilty will probably end up wishing they were arrested publicly rather than dealing with the security service.

TF are you talking about "Vatnik"?

Absolutely!

I find in life it's generally useful to not take things at face value, rather, ask what set of situations and motivations would make people say and do the things they are saying and doing? Asking that question a lot about a person and looking at a lot of their words and actions tells you an awful lot about a person, taking people you know at least something about and asking that about how they are acting tells you a lot more about that situation than you'd get at first glance.

So here we have Zelenskyy, a person elected on a platform of fighting corruption, and an anti-corruption unit whose official assigned mission is to find corruption and root it out and punish the corrupt. That SHOULD make them allies if not friends. And yet, Zelenskyy has just transferred the bulk of their mission away from them, to the security service. He's also elevated the level of crime that corruption is classified as, and thus also the level of investigation and punishment it would carry.

So I ask, why would he do such a thing? Why would an anti-corruption President transfer anti-corruption activities away from the anti-corruption unit and to the security service? What is the motivation for this action?
And why does the anti-corruption unit respond with such a serious accusation (that the motivation is to cover up corruption of the inner circle)?

So let's start with the first question- why did this change happen?
The most obvious answer to me is that Zelenskyy feels the anti-corruption unit is not doing their job well enough. Specifically, if corruption has infiltrated many levels of UA government, that he may feel the anti-corruption unit is itself corrupt, or that they are otherwise ineffective and are not finding enough corruption fast enough. So he gives the job to an agency that 1. has no history of dealing with corruption and thus potentially is less likely to itself be corrupt, and 2. has very sharp teeth as it is used to dealing with much worse things than domestic corruption. And he gives them the order to bite hard- by framing corrupt government officials as traitors and ordering them charged accordingly, the security service has a real mission that they will take seriously and assign good agents to.

And then, why does the anti-corruption unit speak out in this way?
Well they obviously don't like losing stature, and perhaps they are just pissed off at potentially losing their jobs. So it could be plain old revenge- throw some accusations at the President who spat in their faces. But this feels a bit sharper than that. They didn't just say this was a bad idea ('why are you benching the most talented anti-corruption agents in such a critical time?'), they are throwing a specific accusation ('this is only happening so corruption of the inner circle can avoid being publicized'). That's a pretty strong accusation.

So I look at these questions, and ask what situation would cause both Zelenskyy's action and the anti-corruption unit's accusation in response? I come up with two possible answers.

1. The most likely one to me seems that the anti-corruption unit is either itself corrupt or is generally ineffective, and thus Zelenskyy is unhappy with them and that's why they are losing the assignment. And so they fight back hard, probably because (if they are corrupt) they know the security service will find that out, so they want to at least muddy the water first and make the whole process seem politically-motivated and itself corrupt. That way when they are themselves accused of corruption, they can claim it's for political reasons, not because they are actually corrupt.

2. The other possibility is that their accusation is valid- that this change IS intended to cover up inner circle corruption or deal with it more quietly. And that possibility has three sub-possibilities.
   2a. Choice A is that Zelenskyy is only acting on the advice of corrupt advisors, who are feeling heat from the anti-corruption unit and have persuaded Zelenskyy to transfer anti-corruption activities away from a unit that is hot on their tails. They feel they can better control the security service and direct them away from their own corruption, so they advise Zelenskyy to do this and he does it because he is focused on the war with Russia and mistakenly trusts those advisors who are themselves corrupt.
   2b. Choice B is that Zelenskyy has identified corruption within his inner circle and needs it dealt with strongly, quickly, and quietly- without creating a public situation that can be exploited by Russia or his political opponents (who may themselves have Russian influence). So he DOES want the security service to deal with Cabinet-level corruption quietly as is being accused, but that won't result in the guilty being punished any less harshly.
   2c. Choice C is the least likely IMHO- that Zelenskyy himself is corrupt, or has decided to allow or tolerate a certain amount of corruption within his cabinet, perhaps from corrupt people who have proven themselves useful in some other way.

So of those 4 choice, choice 1 and choice 2b seem the most likely to me. And given that this anti-corruption unit has been working at their mission for years and there's still tons of corruption in UA, that says to me they are probably not very good at their jobs. Thus, I advance Choice 1 as the most likely option.