Yes. Before I played MCH(I had to level Archer to 26 for Hawk's Eye), I had leveled Marauder and knew what they were. But before that I played DRG and knew what they were. It can be learned by being aware. Or by someone telling you(Before mentor era).
Clearly you are a special snowflake and the game needs to bend down to your special needs to make an exception on some ridiculously easy requirement you can't be bothered to achieve.

Does this make sense to you? Because from a designer perspective, I sure as hell wouldn't trust my players to know anything about a class they haven't touched. This is mostly true for 99% of the player-base, and I seriously doubt you know the roles ins and outs if all you've played is BRD.

In short: Git gud. Being a Mentor is not for everyone, and if you can't be bothered to meet some arbitrary low requirements without going full Q_Q then you have no place being a mentor. Seriously, most of the people I know are baffled at how easy it is to become a mentor.