How is this relevant?
A: But really there’s no reason whatsoever to put restrictions on the smaller weaker country who is being invaded. War is hell.
B: I think there are good reasons to impose the restriction of the Geneva convention on Ukraine, even if is being invaded.
It's an abstract consideration of the moral legitimacy of an invaded country to act without any restriction (according to OC) or not (according to me). Whether it did break or not the rules of Geneva convention is a completely separate debate. Here the topic is: is it reasonable or not to expect Ukraine, as invaded country, to act within the limits of the Geneva convention?
This is a new perspective I was not aware of. Why would they work only if followed by both sides, considering that affect people outside the conflict and do not grant any military advantages? I don't think it works like this that once a rule is broken automatically "is no longer a concern".
Your example doesn't fit, because you specifically picked one that -while constituting possibly a banned weapon- does grant you military advantages. I am talking about thinks like killing war prisoners, killing or attacking civilians etc., which are the subject of the Geneva convention, AFAIK.