Sure, there's a lot of evidence that they were "terra nullius" before being claimed for the British empire. But the Chagos archipelago was inhabited utill its population was compulsorily expelled in the mid twentieth century.
I was surprised to discover that Ascension even has a ccTLD. I guess I assumed that the population was wholly military.
That's because it was created by the same guy who created .io and .sh - British DNS "pioneer" Paul Kane, who clearly had a passion for finding remaining corners of the British Empire that could "claim" a bit of internet land (for his own profit).
ICANN will act in whatever manner causes them the least trouble, which will be to retain the status quo. They have absolutely no incentive to behave otherwise.
It's pretty detrimental to relationships when one side pre-declares their rules and policies, and then arbitrarily decides to ignore them.
I'd say they have a lot of incentive to behave in whatever way their published policies state, regardless of the impact to users of the cctld.
(I don't know what the policies are)
IMO what will probably happen is that ICANN "promotes" the zone to being yet another top-level non-country code domain like .biz or .horse etc. Which is effectively what it is now.
Edit to add:
I don't think the .su precedent is applicable here. The Soviet Union was an internationally recognised state with a population, military, Montevideo Convention duties, seat at the UN, etc. The BIOT was and is nothing like that.