Goodbye FLoC, hello Topics

Google can’t do that because it advertises on web sites with no obvious marketable context.

That doesn't necessarily follow. I think the idea is to base the ads on what's on the web page, not necessarily try to sell what's on the web page. So, for example, if somebody has a web page describing their hikes, you figure out what hikers are interested in and try selling that. Maybe that's hiking equipment, but maybe people who like hiking are interested in their health, so it's a good place for ads for wellness products.

This was the traditional model of advertising. Advertisers tried to figure out what kind of people would be interested in their product and put their ads in the kinds of publications that would appeal to those people. If you bought a car magazine, it would be full of ads for cars and car accessories, but it would also include lifestyle products calculated to appeal to people who liked fancy cars. If you bought a fashion magazine, it would be full of ads for clothes, but also for perfume and cosmetics.

I think that's intended to be the goal of something like topics. The idea is that it lets ad companies figure out that people who are interested in topic foo are also interested in topics bar and baz. That way even if there aren't any obvious marketing angles related to foo, or if there are more web sites devoted to foo than there are advertisers interested in selling it, they can try selling bar and baz on foo websites.