I actually noticed this the other day. I set a 10 minute time limit on Instagram, and while I'll get the "5 minutes left" reminder, it won't kick me out of the app the way it does for other apps. Running iOS 14.2.
Could be a bug somewhere that I'm not being hit by, of course.
Also, could be the article writer using parent-controlled screen time limits and having accidentally(?) unchecked the "block at limit" option in the rule, which would let the phone-user hit the "ignore limit" button for it.
This seems fixable, either on the dev side or with better communication about how the feature actually works. There's no reason to dupe people.