Most mmos are cpu bound in some way.
And some games especially scale better with single core performance in general, but it’s not because they are just like that, but more because devs need to actually learn how to code for multithreaded programs better.
They are gonna need to do it anyway.
There is an article called.
Why Johnny Can’t Write Multithreaded Programs if you are interested.
Especially considering that this is an old game with an old engine, so this argument applies even more, since when this game was created multiple core cpus weren’t really a thing.