Again, you are missing the point. The issue is not whether he could sue. He could. The issue is whether he should. He should not have.
So in your unrealistic hypothetical, yes Mike Tyson should be allowed to sue you. Whether he should would be based on the facts of your invented hypothetical.
I explained this in the other thread:
A butt-hurt sissy indeed.
If he COULD sue, then what is your problem with it? Do you even have one or just whining for no reason. SHOULD he sue? If he were the average person I would say no. But he is/was a famous public person and someone attempting to generate book sales over an (supposingly) imaginary fight that never occurred based on his name and public awareness of who he is should be sued.
If the average person wrote a book about themselves, it wouldn't generate much interest outside of their friends and immediate family. If that person claimed he whipped Mike Tyson, then more people would want to read it and their name recognition would increase. Doesn't matter if the proceeds goes to charity. They shouldn't be allowed to lie at the expense of another person's name, reputation and character.
In this case, the jury obviously felt the fight story was a lie and judged accordingly.