Everybody does mask to a degree, yes. Absolutely.
The difference for a ND person (speaking as one) is the cost to you of doing it.
An NT person, after a day of putting on for example a professional face at work in the face of dealing with things they don't like or find irritating might be that they feel a bit wiped out or annoyed when they get home. You might work in a noisy office, and the sound irritates you - you much prefer to work in a quiet space but you can still function in that environment even if it makes you feel cross. You get home, you kick off your shoes, sit on the sofa and think 'well that was a shit, annoying day thank god it's over'. You chill, relax, get over it, forget about it.
An ND person, on the other hand passed that 'feeling a bit cross' stage that at NT person reaches by the end of their day eight hours ago. By 9:30am they're already at the point where, taking a classroom as an example, the normal noise of a busy classroom with multiple voices talking, chairs squeaking on the floor, doors slamming, teachers teaching all whilst trying to do their work, all with no ear defenders or adjustments made for them makes them want to rip their own face off with frustration/overload.
They perhaps don't have typical social skills so every word that comes out of their mouth is considered mentally more than it would be by others, who are able to just converse without effort. They have a running commentary in their head 'is that a weird thing to say, have I said the wrong thing?'. They may have to concentrate, all whilst doing everything else, to ensure their facial expression matches whatever the situation demands. 'Is it time to smile now? Was that a joke? I didn't get it, but should I laugh now, because everyone else is. Oh too late, I laughed once everyone else had stopped and now I'm embarrased'.
They're also dealing with whatever transition is coming next, maybe dealing with a new person if they have a supply teacher that day, perhaps seating arrangements have been changed so they've got a new desk partner who they notice - because they notice things like this acutely - smells a little different to the last person, or who taps their pen repeatedly or hums. Their uniform might be irritating them too, labels, seams, maybe their collar is itching them and it's all they can concentrate on. All causing massive anxiety, overload and stress, all whilst trying to put a 'normal' face on. They can't function properly, or if they can it's because they're masking so hard to fit in, avoid being told off, be 'normal' they by the time they get home they just explode, shatter, can't cope. The cost to an ND person, masking day after day, is huge to mental and physical health.
Hope that explains it. Masking is a normal thing, yes but the cost to ND people is far from it.