I would not call it a myth but I also do not accept it is a foregone conclusion either. There is evidence (and a lack thereof) that supports both sides of the argument.
I know many users who use their laptops as desktops, and keep them plugged in 24/7 for weeks and even months on end and their batteries have lasted many many years. My own personal experience with my 12 year old 17" Toshiba was that way. The first year I owned it, I was on the road a lot. But after that, it stayed plugged in and still worked fine until I retired the laptop. It originally got about 4 1/2 hours of run time when new (17" displays are power hogs) and by year 12, it was down to a little over 3 hours. But degraded runtimes due to age is normal.
Anecdotal? Yes, but really, that's all we got here, on either side.
I only used maybe 100 actual charge cycles on it, because I used it plugged in most of the time... should have around 2000 cycles
I agree you definitely should have got more than 100 charge cycles. But how do you know you just didn't get a bad battery? How do you know it died early because you used it plugged in most of the time? You don't. That is NOT a criticism, but an observation. There is no way of telling why it died. And that's part of the problem. I am not aware of any actual study where these failed batteries were scientifically analyzed and determined to have failed simply and only because they were kept fully charged all the time.
And FTR, the claim is not that it will shorted the life because it is "used" plugged in most of the time. But that it is left plugged in and fully charged most of the time - even when not in use.
Also, regarding that article, it is misleading. For example, it concludes,
Stop charging your laptop past 100% today, so you can free yourself from being plugged in tomorrow.
That is misleading because no charging system should ever charge a battery "past" 100% charge. It that happens, something else is wrogn. Also, many laptops today have smart charging technologies where they allow the battery to deplete to some percentage before it charges it up again.
I also have some doubts when it comes to the documentation provided by laptop makers about charging practices. I am certain the corporate shysters... err... legal teams had a say in that so they could cover all their bases to avoid getting sued in the extremely rare chance a plugged in laptop catches fire. Similar clauses are in the warning pages of almost every electronic item we buy today.
I think, just to cover our bases, if you typically leave your laptop (or other portable device) plugged in 24/7, then try to get into the habit of once every week or so, unplug it and let it discharge. Then plug it in again. Do that every couple weeks and I suspect you will enjoy many years of life out of the battery - ASSUMING it is otherwise not defective.