I’m too tired to come up a with a good refutation, though I will point out that it really doesn’t matter either way. If we use his definition of an unstoppable force being an object that can’t have its acceleration altered, using your idea that acceleration is relative to the viewer would have no impact on anything else he said.

For instance, if you try to argue that you could simply change your own acceleration to change its, then it was never truly an unstoppable object in the first place, now was it (in fact, that’s essentially his argument for why an unmovable object cannot exist, which, as he pointed out, is the same thing as an unstoppable force)? In which case you haven’t satisfied the conditions presented. And if you skipped that argument, then everything else he’s proposed would still apply in the same way it always did.

More or less, either neither can exist, in which case it doesn’t matter, or both can exist, in which case it’d play out exactly as he said at the end.