Absolutely, I don’t dispute that.
But what I am saying is that the “how” is still a metric you can measure as a raid leader and based on your requirements, using certain tanks may make a boss or fight mechanic more trivial allowing you to be a bit more creative on your roster as a whole; e.g. beat an enrage timer.
Take Mythic KJ as an example, there are a number of reasons mythic groups took guardian druids. That’s not to say that other tanks couldn’t do it, but running that tank composition just simplified the fight.