I think they're just saying that it doesn't matter if the end user converts based on the ad, but rather whether the company making the ad decides to put it on FB. FB wants to optimize for companies buying ad slots from them, not whether the end user is influenced.
Attributing conversions to ads is a really difficult problem. There's probably a decent amount of cluelessness and institutional inertia that keeps them spending on ineffective ads. See the recent stories on Uber drastically reducing ad spend without seeing any downstream effects.
That's the dumb ML practice of those companies, not Facebook. Also even an irrelevant ad probably converts through brand exposure alone. They are forced to fork over to compete for limited exposure.
