Solipsism is a form of monism: all solipsists are necessarily monists.

Metaphysics is a vast subject in philosophy dealing with questions about existence, like what stuff exists and what it means for stuff to exist.

Most people are dualists of some variety in their metaphysics. They think in terms of the ideas of Self and World as being two different things. Some things would be part of themselves and some things would be part of the external world. This is the normal default Western way to think: It is intuitive, commonsensical and I personally endorse it.

A major alternative to this type of dualism is monism, which is the idea that there is ultimately only one thing that exists. A monist would say that this distinction between Self and World is ultimately an illusion: that the self and the world are one, and that all things are one. Monism is kind of a big deal in Eastern philosophy and you may have encountered it on a popular level in the half-baked pantheistic philosophy of the STAR WARS movies, where everything is one because the Force. Yoda's a monist: Han Solo's a dualist.

The point I am making here is that a solipsist is a monist by logical necessity. If the self is all that can be known to exist (definition of solipsism) then there is only one thing (the self) that can be known to exist, and that's monism.

I myself am not a solipsist of course, which if solipsism is true means that my being wrong is all your fault.