Solipsism is incoherent because it's not radical skepticism. All of the critique of the external world also apply to belief in the primacy on internal experience. Any good solipsist should just accept the "evil demon" of descartes, embrace radical doubt, and say "I don't even know if I truly exist or not".
"I don't know if I'm a P zombie, and I don't know if I'm a replicant or not, Deckard!"
"I don't know if I'm a P zombie, and I don't know if I'm a replicant or not, Deckard!"