Yep, at 0.1-0.2% (1000-2000ppm) people start to get drowsy. at 0.2-0.5% (2000-5000ppm) people get heachaches, poor concentration, elevated heart rates, etc. It's quite unpleasant. 3% is 30,000 ppm.
While wearing a mask isn't fun, I've never experienced anything to that level while wearing mine.
I'd caution you against this idea that you can "know the feeling" of acute CO2 exposure.
For my PhD work (gas geochemistry) I spent hundreds of hours working in volcanic ice caves with CO2 levels up to 2% (we measured up to 3% but had a rule against being in above 2%). I always had a gas monitor with me so it was interesting to try and guess the levels. Neither I, nor anyone I was with, could guess reliably.
More than once I was with others in a cave, and they would freak out and say they felt high CO2 and we'd check the readings and nope. I actually think people (including myself) were responding to high humidity and heat and misinterpreting it as high CO2.
Of course, high CO2 is dangerous, all I'm saying is that it's surprisingly hard to disentangle it from all the other factors affecting how well your feel as a human, when you actually test yourself.
I'm reasonably certain I notice levels above 800ppm. It's subtle, but everyone in the house starts getting a little edgy (kids and adults). Sure enough, the CO2 meter is reporting > 800.
I doubt I could differentiate between 800 and 1600 though.
In the early days of COVID, before we knew anything, I would go for runs outside and wear a mask (N95). I live in a densely populated city so this seemed like the appropriate thing to do.
While it was not pleasant, at no point did I start having issues you described so I have trouble buying the original claim.
While wearing a mask isn't fun, I've never experienced anything to that level while wearing mine.
Not buying it either.