That's why the knowing caveat. E.g. either you HAVE to advertise to your own users (so if your browser is already the default, you still have to show the notification to suggest to change), or you are treated as if you know, and then you can't advertise based on the 10x rule.
To put it differently you can't hide ad for Chrome users and then pretend you did not know others used FF.
How would the regulator know when you break the rules though?
I may very well be misunderstanding the idea here, but it still seems like it would come down to (for example) Microsoft being required to limit how they send user notifications/recommendations based on Microsoft's own internal analytics for user metrics.
For the Chrome example, the regulator would have to know how Google's proprietary ad targeting algorithm works to confirm that it doesn't target based on user agent or browser manufacturer. I'm not sure how the regulators would really be able to pull that off any better than regulators asking VW software for VW diesel emissions measurements.
Users will report it. 3rd parties could collect statistics.