How do you even know he actually means it

I would recommend that you tell her. As his dementia gets worse she will constantly be treated to stories about the other women as he forgets what he can and can't tell. Do you really want her to be subjected to years of this humiliation? Also, with this situation, it is inevitable that she will find out. You don't want her to also find out that you helped him keep this secret. Yes, there is a possibility that she will kick him out but he made his choices and choices have consequences.

Obviously shitty of your dad, but new siblings could be cool!

The truth is you have no way of knowing at this point if Dad has multiple other children or if that’s just a delusion. If it were me, I would assume what I think is the best case (?) and assume Dad is confusing reality with what he might have imagined. Real or not, I wouldn’t be the one to break the news to Mom. This is a terrible situation for both of them and there is no reason to make it worse for Mom if this were real - especially if there is no actual proof. By the way it’s completely pointless to argue with someone with dementia or delusions. You will never convince them, and even if you did an hour later they won’t remember.