I doubt they deprioritize websites, but if they over-prioritize sites that conform to their SEO guidelines and host their ads, then the net effect is the same.
And I don’t see why a quality website wouldn’t keep a viewer on site as long as a poor quality website.
Say you ask a question looking for a definitive answer (meaning it's a question that has one). You find a website that tells you that answer right up front. No fluff, no scrolling, no ads, you're in, you're out. You find another site loaded with a introduction that talks a lot and says little, then you scroll past some ads, then you find another paragraph of text, and somewhere buried in between wishy washy word salad, you find your answer.
To Google, the second website is better, because you were "engaged" for a longer amount of time, and maybe you clicked their ad as you scrolled. That page is gonna rank higher in their results. The former page, by consequence, is gonna come second.
Now let's say you have 10 websites like the latter, and 1 like the former. What rank is the former website gonna come in Google search?