I think we're "supposed" to destroy ourselves for the sake of advancement for a few reasons:
To preserve the myth of Western egalitarianism. Supposedly, we have a classless society. Anyone can make it if they just put in the effort. Mind you, this isn't true: plenty try and fail, and even those who succeed sacrifice their life to advance from one class to another. But we're supposed to believe that the only reason we don't have certain things is because we don't want it bad enough, and/or lack the discipline to succeed. The goal: get people to always look inward for the source of their suffering, and fail to recognize the very real economic parasitism that prospers at our expense.
A manifestation of that old but persistent notion that to be righteous is to suffer. If you are happy, if you aren't suffering, you must be doing something wrong. Good food tastes bad. Good exercise hurts. Good work is miserable. To be good in spirit is to mortify the flesh. Put on your hair shirt, run five miles, drop and give me twenty, and then complete a twelve hour shift. Sleep is for the weak.
What offends people who take this advice more than anything is someone who hasn't lived this way, and yet is happy when they are not.
I think the main reason people say it is because growth happens during times of discomfort. If you're only ever comfortable, you'll become weak and die early because your body will fail faster.
My sister was diagnosed with ASD prior to transitioning, and she went the complete opposite way from the "keep yourself comfortable" mindset.
It can absolutely be fun to be comfortable for a while, but eventually the other shoe will drop and it's going to lead to misery and deteriorate your mental state.
I have a roommate with ASD that I've really been trying to drive this home to, and completely honestly, I feel incredibly guilty like I'm enabling him by allowing him to live with me for cheap without imposing any expectation that he improve himself.
Discomfort stimulates growth, but the actual growth happens during periods of recovery. That is true of the body, and I have little doubt it is true of the mind, as well. I'm not saying people should never step out of their comfort zone. But just like we shouldn't be judging people at the gym because, from our perspective, they should be able to do more, we should be extending compassion to those of us who have difficulties in the mind, particularly considering we can only know our own perspective, not theirs. I mean, you wouldn't expect a guy in a wheel chair to be doing leg presses, would you?
There is an appropriate time and place for one to leave their comfort zone and this differs for everyone. The advice levelled at people telling them to leave their comfort zone, is at best, tine deaf.
I agree with the other commenters that you should spend some time outside your comfort zone but pace yourself very carefully. The article mentioned a proximal zone that's outside but adjacent to the comfort zone. I think it's good to mostly shoot for that with occasional planned forays further out just to test yourself.
IMO nobody should be telling you when to go outside your comfort zone or how far, or whether you're doing it enough, unless it's someone close to you whose opinion you can trust and who you know will hear you out when you say you're overwhelmed. And even then you still have veto power because you're the one who has to deal with the fallout if you push yourself too far and melt down or burn out. I don't ever see any NT folks volunteering to help people clean up their life after that except maybe social workers and therapists.
The flip side of this is that since nobody can tell you how much is too much, you're responsible for monitoring that yourself and communicating or removing yourself before you get overwhelmed. That's a good use for the proximal zone - testing your boundaries and keeping an eye on your mood so you can learn to spot when you are approaching your limit. Easier said than done, but I've found it worth the effort. And it gets easier with time.
The other thing to recognize is that some days your comfort zone is pretty big and other days it's about as big as your bed. Asking yourself "how big is my comfort zone today" helps you give yourself some grace. If it's a bed day and you got out of bed, you already exited your comfort zone and should factor that in when you plan your day.
Too much text so my brain won't let me read it and thus this might just be a summary of what you said, but
My take is to treat it like exercising muscles, you push as far into discomfort as you can handle (never reaching pain, pain is a sign of damage rather than growth), then you spend a long time resting and recuperating until you feel ready to push it again.