They haven't. It's really no different than it's always been -- judges have varying degrees of experience, some have drank/brewed/judged most or all of the styles, and some aren't even close. Most judges strive to do well at any category they are assigned to, barring physical or sensory issues.
(For example, I cannot judge smoked beers, my palate is absolutely shot two beers in, so I always ask not to judge them. I won't do a good job, certainly not up to the high standards I place upon myself, so why bother?)
And I've long held that any moderately experienced judge should be able to judge a style they've never had in their life, simply by using the guidelines. Part of being a good judge is being able to interpret the guidelines and "connect" them to what you are perceiving in the beer in front of you. It's not ideal, we should all strive to gain experience with the full range of beer styles, but it's not always possible.
In regards to mead and cider, the BJCP have separate certifications/endorsements for those now, and one can become a mead-only or cider-only judge without ever taking the beer exam (although most judges added those certs to their existing BJCP rank -- for example, I am National + Mead).
And to add to the confusion, you do not have to have either the mead or cider certification to judge meads or ciders, taking those exams is entirely optional. (Most, but certainly not all, organizers will try to fill the mead and cider flights with certified mead and cider judges, but it's not always possible.)