the argument that “being selfless is selfish” is not useful

Yes, that’s my entire point.

and provably false

Depends on how you define “selfish”. Again, that’s exactly what I’m trying to demonstrate here. Reducing the definition of selfish to mean “getting something out of it” makes it meaningless because every decision is made in the hopes of getting something out of it in some way, even if it’s obscure. To make it useful, you need to look at what someone is getting out of it in order to get to a useful definition.