Like most words, capitalism has multiple definitions. Among the popular ones, the one that is about capital doesn't concern itself with markets, only capital, so you are quite right that any kind of market goes. It could even be centrally planned! But another popular definition is about the "invisible hand". Rent seeking is absolutely considered to be at odds with the "invisible hand". This is most likely what the parent is talking about.

And no doubt there are a bunch of other definitions that aren't so popular, so the parent commenter could even be using one of those. It might even be his own pet definition that he just made up on the spot right now. The author always gets to choose what a word means, so if something seems off "It isn't" isn't a logical retort. You first need to clarify what the author intended the word to mean.