When and where do they say this?
Literally right at the beginning of the playtest packet, in the sidebar marked “this is playtest material.” Here’s the relevant bit:
Power Level. The character options you read here might be more or less powerful than options in the Player’s Handbook (2014). If a design survives playtesting, we adjust its power to the desirable level before official publication. This means an option could be more or less powerful in its final form.
And assuming it is accurate why do they bother with a public playtest at all other than as theater?
To get a sense of general interest (or disinterest) in the options presented. They want to know the audience’s broad emotional reactions to the concepts. They work out the design specifics between their designers and their private playtesters.
Seriously, if the public playtest doesn't have enough information to actually judge whether a thing works, it is essentially useless for design -- which means the public playtest isn't about design, it's about "feelings" which literally no one responsible for a revised edition of D&D should give a single solitary naughty word about.
I strongly disagree that the designers shouldn’t care about the audience’s feelings. That’s… kinda the thing they should care about the most. But, yes, you have correctly identified that gauging feelings is the purpose of the open playtests.
It's disingenuous. Even worse, inviting the current fanbase to give their feedback with no intention of it mattering is predatory.
It isn’t disingenuous at all; they are very open about the fact that they are gathering feedback on the concepts, not the specific design. The feedback does absolutely matter, because if the general response to an idea is too negative, they’ll change or (more likely) drop it.
I think maybe they don't care what their fans think and will do whatever. Which is fine and is the way design should be done. But it's pretty naughty word to invite your fans in just to market to them.
They do very much care what the fans think. Or more precisely, how the fans feel. The lesson they learned from 4e is that feel is paramount. But no, they don’t care about fans’ design ideas, and have never really claimed they do.