Attribution would be as easy as knowing which chatbot they came from? Unless chatbot publishers (openai in this case) allows you to target based on the attribution, that extra data will not be actionable, anyway.

Ads are placed in ways that don't directly connect to a sale already. Billboards, TV (last decade), youtube sponsors, even at least half of traditional internet ads. Generally they'll just see whether sales increase after they start running the ad campaign, if so, it works.

That's missing the point, I think. With those, you (as an advertiser) can verify the ad yourself. With an LLM, especially a subtle one, you can't.

Just ask it the best brands of X

In every single case you've mentioned you're going to have a campaign with an attempt at attribution. Certainly youtube sponsors all use direct attribution with "use my code" etc.

In every one of those cases you can at least do things like count viewers. How would that work for a model that's just subtly biased?