The maxim is 'if you're not the customer, you're the product' not 'If you're not the customer, you're the product we sell to advertisers.'

It's a pithy quote and nothing more, one I can disprove handily by pointing at any number of completely free, donation based services online.

You are not being "sold". That entire line of thinking is intellectually lazy and meant to provoke an emotional response. I wish people would stop it.

Absolutely agree, it's a terrible phrase.

No, it's disingenuous to suggest YOUR content is unrelated to the value of THEIR product.

A reasonable analogy would be conferences, are you paying for the venue, or the access to other people/speakers? does that mean the people/speakers are the product?

My contribution being of value to the site is one thing. Tracking that contribution and using it to target advertising is a completely different thing. Suggesting otherwise, as you and others appear to be doing, is deliberately disingenuous.