FWIW, the passive voice is completely natural in spoken English. Otherwise, you wouldn't have admonishments against it in style guides and English teachers wouldn't be slicing through instances of it with a red pen (or modern equivalent).
The author is a linguist where passive has a technical definition and implicitly wishes that people would use some other word for what they have an issue with.
Uh I mean he's a linguistics professor. People are misusing "passive voice" when describing something they don't like and of course the linguist is going to get ornery about it. If you have something against clauses with diffuse agency say that- don't put the blame squarely on passive voice.