I think it's somewhat more important than that. Basically the point is that the A/B tests tell you "which ad is more effective spending on this particular platf

I think it's somewhat more important than that. Basically the point is that the A/B tests tell you "which ad is more effective spending on this particular platform with the particular arrangement of users you selected". If you try to expand the user group after the A/B test is over, or if you take the ads to another platform, you shouldn't expect the results to hold. So if you're an ad exec, you should make sure you run separate A/B tests for each specific ad channel you intend to spend money on. An A/B test on Facebook does not tell you anything about how well the same ads would do on Google, and even less so on TV (even assuming you are reaching roughly the same type of audience). This happens because Facebook's targeting mechanisms are not the same as Google's, and so they may optimize the ads differently than Google within the same population groups you selected and give you different results. And TV ads are not targeted at all, even if the audience you are reaching is the same.