They know, and they're probably proud of it - or they at least enjoy the sound. I say this because I myself use a loud mechanical keyboard (but I work in my own space so it doesn't bother anyone).
The noise is probably due to the type of switch they are using, if they are as loud as it sounds, they are probably using a switch from the Cherry company called and MX Blue. There are silent, or at least quieter switches out there, but changing between them typically requires getting a different keyboard.
As for changing their behavior, that could be difficult. You could ask them if they have a keyboard that uses Cherry MX Brown switches that would produce less noise. If nothing else your knowledge of the niche interest may impress them enough to give you some social leverage to induce a change.
Ultimately it doesn't seem fair to use a loud keyboard in a shared space if everyone present isn't on board with the noise level. You should talk to them, and to your manager and see what kind of compromise can be struck.
Of course, you could also try a mechanical keyboard of your own... Perhaps you'd become a fan of the satisfying clickity-clack your coworkers enjoy...