Solipsism is skepticism of the existence of anything outside the self. I've seen Hoffman address accusations of solipsism a few times and I have to admit that it's always been unclear to me which part of his theory people tend to perceive as such. Perhaps I've just consumed enough of it to zoom past this perception.
I'll try to keep things short, as this can get pretty long winded fast.
From what I understand of his proposition, it's a take on idealism that is very close to eastern thought as inspired by nondual traditions like Advaita and Buddhism, but with a heavier emphasis on science. Everything in reality is a projection in consciousness of consciousness. It's made up of interacting conscious agents (you, me, a rock, an atom, a particle, etc) which are themselves "projections" ultimately stemming from a fundamental, unknowable, infinitely distant and unattainable root conscious agent. The implication is that space-time, our perceived reality, is not fundamental. Hoffman thinks that we might possibly have access to at least one, higher, more general dimension of reality of which ours is a specialized version (as hint of this, he speaks of current work in physics where structures outside space-time are being discovered, like the amplituhedron).
Space and time not being fundamental creates problems with some materialist assumptions in evolutionary biology, where consciousness is seen as part of an evolutionary process. Hoffman suggests to rethink evolution from scratch instead. He uses evolutionary game theory to demonstrate that we can have consciousness as fundamental, keep some of the core evolution principles and still end up with consistent conclusions.
I'll stop here, as I've said, it can get deep rather fast.