I think it's pretty easy to define, actually. Were they paid in some way to do those things? If yes, then it was advertising.

It sounds like the most common way to do these things is to have one company operate one gambling and one non-gambling site and just tell people they operate the other site on each. No money's changing hands, so that's not advertising. Then you can advertise to go to your non-gambling site, and they can organically navigate to the gambling site which was disclosed, not advertised. You would almost have to ban companies which have any interest in a gambling product from advertising anything at all.

That sounds like a conspiracy and the penalties for conspiracy are much more severe than just illegal advertising.

Conspiracy to do what? Advertise? We already established it's not advertising.