Unless lemmy.ml decides to unfederate from lemmy.wtf you should be able to subscribe to any community on lemmy.ml using your lemmy.wtf account. And it even appears like lemmy.wtf already knows about the community on lemmy.ml. Otherwise you would have needed to search once for it (from lemmy.wtf) using the full URL.
Well, you appear to be logged in on lemmy.wtf and you don't need to be loged in on lemmy.ml to access (subscribe to) their GW2 community using the link (This is your local copy:) above.
Maybe federation (syncing) doesn't work properly at the moment. Many instances kinda struggle at the moment due to the high number of new users joining the fediverse (leaving reddit).
Sometimes this takes quite a while and when you refresh your browser window (while the "busy" animation is still playing) the status turns to "Subscribe Pending" (yelllow). From my experience this already counts as a "success" and will eventually turn into "Subscribed" (green) later.