Think of it like a MOBA. There are basics and fundamentals that apply across the board - if people don't 'get' them, it won't really matter what they play as the weaknesses will still show regardless. It doesn't mean they're terrible players, just that they have blatant gaps in their game. In a MOBA, positioning is one basic (but vital) factor whether you are a tank, support or damage dealer - people who aren't good at it won't do very well with any hero unless carried. The amount of players out there that couldn't position their own underwear is exceptionally high (and it's cute how so many commit proverbial suicide and then choose to blame everyone else for the fault they can't see). Much like positioning in a MOBA, it's no different to enmity/threat in a MMORPG - you don't need to have played a tank to know the basics of enmity. Even first-time MMORPG DPS or Healers have time to realize that them having aggro in threatening content is typically a big Aww-Hell-Naww.

Now, let's say you've mained X tank for Y period and decide to swap - yes, the abilities are different and it likely has a different rotation or mechanical aspect, but the fundamentals are pretty much the same. A good [insert tank here] who understands enmity and when/when not to use defensives (or just plain and simply knows the content they're walking into), well, they could likely swap to a tank they've never played before and still do a decent job outside of class inexperience. Certainly won't play them perfectly and probably won't be entirely comfortable (initially), but they'd know what needs to be done and typically how to do it, given time to figure out which buttons actually do what you want.