The point, if I understand correctly, is that WP is libre and nominally gratis, but it has become so complicated (and resource-heavy) since its conception as a blogging platform that it often costs real money to get it up and running satisfactorily. And there is a certain truth to that; it has, to an extent, become consultantware at industry scale, and often forces even "lightweight" personal sites onto more expensive hosting plans due to file loads, database hits and memory usage. There are "standard" plugins that can help a lot, but that's pretty much opaque to the typical non-technical end user, and it takes more than a passing knowledge of, say, HTML+CSS to style/theme a WP site. Free themes, which a user will often turn to after giving up on the idea of learning to write their own theme are often multiply base64-encoded (and just as often are ad-plagued and/or malware-harboring), and after being bitten once or twice, people resort to paying for themes or to have themes created for them. Not much, perhaps, but more than the "free" (gratis) software they thought they were buying into.

The author has the saying backwards.

WP is free as in beer (you don't pay for it) but not as in speech (you can't do whatever you want with it because of the GPL).