The thing is, the "natural selection" of this network is decided by people on average spending 5 seconds of thought before getting outraged/scared/intrigued etc (also, whatever is that emotion that says "look world, I found something really clever that y'all have missed!") and clicking retweet. I find that incredibly unlikely to yield any great content whatsoever.

It's a clever trick. Short tweets with pictures skip the rational thinking part of the brain, for those tweets don't have anything to think about, and go straight to the emotional animalistic part of the brain. So twitter turns into a sort of colossus stadium where millions of apes screaming at each other. Usually, the stadium splits into a few big chunks where apes chant the same meme.

Advertising is placed into the feed, but it doesn't determine it. You still get what you like and engage with the most.

I'll believe claims of this sort when production feed generation algorithms are released under an open license.

What difference do you think the feed makes for slotting in ads every few stories?

I believe that the feeds are far more manipulated than this. Facebook got caught once intentionally manipulating feeds to attempt to produce negative mental states. Do you honestly think they just completely stopped? I think it's plausible that it's happening on a much larger scale than it was then.