I feel like people who talk about "not looking at your bar" must lack peripheral vision or something. That, or they're the kind of player that's obsessed with "minimalist" HUDs, where all useful information gets shoved into a corner.

You can look at your bar. You can look at boss mechanics. You can do both. Mechanics don't happen in the blink of an eye. Especially not with server tick nonsense. And along the same vein, combos are usually pretty consistent for a handful of GCDs. Looking at one or the other for 2-3 seconds will not kill you.

Besides, all that muscle memory you've built up against a target dummy? It gets thrown out the window as soon as unplanned downtime happens, or you drift literally anything. Which is virtually guaranteed to happen in a fight that you aren't familiar with. So, muscle memory ends up being useful only when you're already familiar with, which kind of means you already know the mechanic that's about to come up and what to look for.