Like you kinda hear this a lot with the debate about which genres are harder than the others where people will unintentionally argue which types of skills they think are the hardest without realizing it, because they don't consider certain things to be as hard as they are.
A shooter that has fast movement inherently promotes twitch aiming and accuracy above other skills, a shooter with slower movement promotes map knowledge and enemy tendencies to know where people like to be and what your opponent favors so you can get the drop on them.
Both styles of shooter have these as skills required to excel, but different designs push which skills have higher emphasis. The slower game isn't inherently easier because twitch aiming is less of a focus, the higher amount of knowledge and adaptation required can be much harder.
In general people tend to think physical feats are the height of difficulty because there is a literal barrier to them, some of which a select number of players will literally never overcome, but the reality is that physical skills are just one type of skill, and not inherently the most challenging.
Just as some people will never climb the hurdle of certain physical skills, there will be tons of people who are not capable of keeping certain things in mind at all times, like mentally tracking where an enemy is or keeping an item spawn timer on rotation permanently.
Noticing the plays your opponent likes doing, keeping them in mind, countering them, and then being on the lookout for when your opponent switches up their tactics so you can react accordingly is an incredibly hard thing to do. The majority of players don't even consider this at all.
Of course, at the highest level, the best players can do all of these things to varying degrees, some being better in some aspects than others, but we're talking about 0.1% of the player base for shooters. 500 out of every million, maybe. Even among these players, no one is the best at everything.
That's why the pros of one shooter are not the best players of every shooter. You can't just say "Well Quake is the hardest shooter of all time because it requires the most insane twitch aiming of anything, those guys should dominate CS," because CS has so much required knowledge to be good at.
Quake guys would dominate the ARMA scene
These skills might in some sense transfer from game to game, but because each game emphasizes different skills, being good at one game does not mean you will be just as good at another game. Being a multigame FPS specialist (at the highest level) is incredibly rare. It basically doesn't happen.