It does happen obviously: https://en.wikipedia.org/wiki/Space%E2%80%93time_tradeoff
But yes, running another browser + javascript interpreter on a machine that already has a browser + javascript interpreter running isn't what that article is talking about.
I’m totally with you for things that are actually doing serious numeric/algorithmic work. Matrix inversion runs a helluva lot faster when the whole matrix fits in RAM. And there are undoubtedly lots of places where you can use different algorithms for the same problem that can trade off memory usage and computational complexity.