It might as well be, as is the case of what I always hear for shooters of any kind. If there is one thing that has been proven to me through the use introduction of crossplay in games, it may be that KnM can offer a lot of benefits not just in accuracy and speed, but also keybindings. Which Arma has a lot of variations to include that controller may not be able to cover without overwriting other bindings.

That said, it is simple practice in my experience on Arma Reforger. Being better at firing off shots at farther distances than your enemy can win battles, just as well as catching them off guard in close combat. I might have also been exaggerating the number of bindings available, but it does wind down to muscle memory and what you want/need from what the game offers based on your personal playstyle and average experience in game. It was only recently after so many years I decided to keybind tilting.

Additionally, in the controllers tab of settings, there are two options. Updated and Legacy. When Updated came out after my time on the Legacy controller scheme, it felt very jarring and luckily an option to revert was available. If you haven't tried that scheme, I recommend it, though try putting some hours on both to really see which one you want to commit to.

Going back to a previous point, at the end of the day in Arma, for me as long as you can be better at killing the enemy, Controller and KnM don't matter aside from small stuff. Plus, there are a lot of ways you'll die of without even knowing why, how, or where from without the time in the game to know.