Two things

First, it sounds like this 6gb requirement is more like a suggestion/recommendation than a requirement. I also am curious if it actually actively uses all 6gb. From my own usage of Linux over the years the OS itself isn't using that much ram, but the application is, which is almost always the browser.

Secondly. I haven't used Ubuntu desktop in years. So I have no real idea if this is something specific to them, but I do use Fedora, so I would imagine that the memory footprint cannot be too different. Whilst I could easily get away with <8gb ram, you really kind of don't want too if you're going to be doing anything heavier than web browsing or editing documents. Dev work? Or CAD, Design etc etc. But this isn't unique to Linux.