When you list all artists:
http://www.e621.net/artist

There's the heading "Is this User".
Is it correct that only admins can change this field?

I'm asking because it's not mentioned here:
http://www.e621.net/help/artists
(or anywhere else that I could find)

I guess it fills automatically when an user is also registered here, which I suppose admins associate usernames with artist tags

Admins are the only ones who can link a user account to an artist tag, yes.