I've seen it come up several times now and I don't know why. BPers seem to have this idea that the only definition of solipsism refers to the philosophical theory of mind, that you can't know that anyone else's thoughts and experiences exist besides your own, or that you are the only mind that can be known to exist.

This is a definition, yes. It is not the only definition. Sometimes words have more than one meaning. I can think of shit ton of them, this is not an uncommon occurrence.

It's pretty easy to find the second definition, they usually come after the first. I'll give you a couple for solipsism, just to help you out:

extreme egocentrism

And

extreme preoccupation with and indulgence of one's feelings, desires, etc.; egotistic self-absorption

This becomes even more apparent when you try to use it in a sentence, especially as an adverb or adjective. Since we are not talking philosophy, it's probably safe to assume that those definitions are the ones they are using in that context.

That being said, why is this even necessary to bring up? I identify as RP and I can find a bunch of problems with the theory that are actually worth debating, yet some of you get hung up on a seemingly inconsequential perceived semantic. At most, this is just liberal use of an exaggeration. You can find plenty of worse things said about people on TRP.