It's not meant for that kind of comparison. It's a variant of Simpson's paradox. Any individual system with a fixed set of tasks needs less swap when it gets more RAM. But when you look at the aggregate of systems, the systems that have more tasks to run get more RAM to run them, and systems with fewer tasks get less RAM. And since more tasks need more swap, everything scales together (though often not linearly).

I'm getting tired of typing this, but swap space is not just to increase available virtual memory. If you upgrade from 8 GB to 24 GB, then with proper swap space usage, you have 16 GB that could be used for additional disk cache.

Sure, you're still better off with 24 GB overall compared to 8GB+swap whether you add swap to your 24 GB or not, but swap can still make things more better.

(That says nothing about whether the 2x rule is still useful though, I have no idea.)