I don't know what he's talking about either. Most popular NTP clients (ntpd, chrony) will try very hard to make sure this never happens by simply slowing down or speeding up time. You don't know what will break if you just gap time like that.
ntpd and chrony might do it (I'm not sure), but systemd's NTP implementation (which is widely used, even though it does have many other issues -- such as not implementing the spec properly IIRC) does just jump time when you enable NTP on a system where it was disabled. From memory, back when I used ntpd, it did the same thing but I could be mistaken.
Interesting, might you have any links to these discussions?
Not sure about chrony, since I haven't used it (or heard of it, admittedly).
The whole article is about edge cases. It doesn’t really matter if they aren’t super common: the result is that mtimes do act weird sometimes, and if you build a system that depends on them, it will also act weird sometimes.