It looks like the TLD was sold off to a private business by the Internet Assigned Numbers Authority in 1997, with those rights subsequently being sold on to other corporations.
The British government have issued an FOI response advising that they recieve no funds from .io domain registrations. The Chagos Islanders still don't benefit, but it looks like that'd need to be squared with a hedge fund rather than a government.
...It is weird that territorial domains can be auctioned off in the first place though.
When the ICO recieve a complaint they usually send an initial notification email to the data controller to advise that a case officer will be assigned in due course.
Well, unless it relates to a serious or ongoing data breach, which tends to be triaged immediately into an active investigation.
Initial notification letters do usually recommend trying to resolve the issue with the data subject in the interim though.
That probably spooked Reddit into moving your case up the priority list as I imagine they've got a pretty substantial backlog of SAR, erasure and objection requests, considering the circumstances.
The response window for most of those rights is 30 calendar days + extensions if applicable, so they could also have just been responding as late as allowed, accounting for aforementioned probable backlog.
Do let us know when the ICO gets back to you though, will be fascinating to hear what they have to say.