When you reallocate your array you will in memory have your old array and your new larger array while you move your data over. At the very least you're using 2x and the extra memory for your expansion.

For your other points if you'd mentioned them in the interview you'd probably have been better received. Copying is really only that fast for POD objects (your objects copy constructors may need to do reallocation themselves or worse) so if you're suggesting a general solution you should be aware of that (or at least mention move constructors if they were available at the time) .

I would be surprised if any of the games you worked on actually shipped with an amortised resize of dynamic arrays (at least not for anything that didn't matter in the first place) so I don't know why you'd suggest it as a general solution in a game dev context.