Sure, they thought GTFO (18k peak players at launch) or Potion Craft (10k peak players at launch) were worthy of massive free curated advertising to the Steam community at large? And those are just two examples I could find while doing google searches.
When I ran Enhanced Steam I had indie developers offer me tens of thousands of dollars to add these types of takeover banners to the Steam homepage, just for the small subset of users that used the extension. I had big publishers offer me... more. (I never took them up on these offers). I think a lot of people underestimate just how valuable this ad space is. And I also think it's naive to think that Valve is only doing this to prop up games they think are gonna be "the next big thing" so that they can get their 30% cut of sales. If your counter-argument is just "but he just said it in the video - you can't buy ads on Steam", maybe have some self reflection about the number of times in your life a large multibillion-dollar company has lied to you to make themselves look better than their competition.
Or, perhaps I'm just getting old and jaded. I don't believe them.