You are correct. "Prices continue rising" isn't a grammatical passive. The subject are prices, which are performing the action of rising, all by themselves.

I see in Pullum that this mischaracterization fits example (23) about how BBC's Tim Lewell claimed that "five girls have died" is a passive construction compared to "the man went in and shot five girls".

These sentences displace the action from the the true agent, onto the object. Passive sentences are disparaged for something similar, but not the same: avoiding mentioning the agent.

Actually, displacing the action onto the object is worse than simply avoiding mentioning the true agent.

"Five girls were killed" is passive, but at least acknowledges that someone or something did it to them.

Same with the passive "prices continue to be raised".