Only if you know for certain that the key has been accessed by a third party.

If you don't know for certain, you have to factor in the likelihood that it has been, and at that point, the two risks aren't equal anymore so that logic doesn't work.

What if you don't know for certain ?

You just ignore it and hope for the best ?

Only if you are certain (and better be really sure you haven't missed any cache/cdn, temp files backus etc.) it wasn't accessed you do nothing.

It was publicly exposed, and if they are making this announcement it’s essentially guaranteed they can’t rule out it was accessed.

Are you arguing for the sake of arguing and technical correctness or do you actually believe Github shouldn't rotate their key in this situation?

What? This is a terrible way to reason about risks in general. If you don't know for certain, you should assume the worst case scenario, especially since it's impossible for you to calculate the probability distribution of the likelihood of a leak.

You should only keep moving along without key rotation if you know for 100% certainty a leak didn't happen and no one accessed the key (not theoretically impossible if they had the server logs to back it up), but anything minus that and you have to assume it's stolen.