[chongli]

I think your confusion arises from the fact that this discussion is about two different thought experiments: the brain in a vat [1] (a la The Matrix) and proper solipsism (which implies no real world at all, not even a vat). [2]

[1] https://en.wikipedia.org/wiki/Solipsism

[2] https://en.wikipedia.org/wiki/Brain_in_a_vat