I think the big thing here is that you can buy an already approved additive package, and, when blended with one of the approved base oil combinations, this yields an approved product. Infineum/Afton/Lubrizol already do this work for you if you aren't XOM, Shell or BP and aren't in the business of developing your own ground-up additive packages.
Now, if you take that same additive package, and blend it with some better base oils (so, PAO instead of Group III, some AN, some ester) are you totally erasing the validity of all that testing done by the additive manufacturer? After all, you aren't changing the fundamental chemistry of the additive package.
This is a very different approach from say, Redline, where the additive package is not one that was approved, and consists of very high levels of certain additives.
But I agree that, generally, it's best to see both certifications AND good oil engineering. It does however seem that the API doesn't set the bar very high, which is why many OEM's have their own approvals, particularly the Euro marques who don't give a 2nd thought to the API and generally have their own, extensive, testing and approval process.