I'm not sure that's true.
Advertisement campaign a, takes people to landing site b.
Anybody who shows up at site b, you know was targeted and campaign a.
And if you're saying b is some generalized large domain, I promise you no advertiser would ever do that. They would set up subdomains, or campaign specific domains, any landing page where they know where you're coming from. And there's no way to stop them from doing that
At its core, most online advertising is just about a campaign to send people to a location. And if you can specify the campaign, and you can specify the location, you know which people came from that campaign and what they're advertisement factors are