I see it more like a question than a rule.

"The service is free. Am I the product?"

That is a valid thing to ask. Even with FOSS sometimes.

Some FOSS projects are backed by companies, then yes, plausible to ask.

Otherwise, I would answer with a clear no.

(Projects can still collect telemetry and other data and sell that, though the sell part should be very rare, imo...)

Edit: Was that a bad faith argument or a honest question?

Sometimes I can't tell, maybe because of old or ESL...