Depending on when you pick up the third job, you could easily get three to 90. I leveled dragoon and reaper to 90 and still had tons of content left for a third, but reaper is a level 70 job, so it eats up less exp than something you'd level from 1. I hit 90 on two jobs before Endwalker even started and I was going pretty fast through it. I've still only been playing a few months. I think some people that have been playing a long time and never leveled a character up through all the expansions from the start don't really realize how much experience you get doing the entire story, honestly.

I think there's also an attitude that you just need to get everything unlocked as soon as possible because it makes a lot of the game easier when you have all the maps and can fly everywhere, plus more duties, etc, unlocked. I definitely don't think you need to skip cutscenes and stuff to get there a matter of hours earlier, but they're not wrong that shirking the main story is going to make you miss out on a lot if you're taking ages to get through it. You'll end up not being able to join your friends on treasure hunts, instances, etc, because your content is still locked. I think the idea is that focusing on one character would push you through faster, but that's not necessarily true. You could go through just as fast on multiple and not end up just flushing exp down the drain as long as you're not spending days grinding to keep up tons of side jobs.

Basically, it probably isn't efficient to try to level 4+ jobs because you'll need to put off doing story to keep them all on pace, but it's probably more efficient to level around 3 (possibly one you pick up at 60+ -- Dancer, Gunbreaker, Sage, Reaper) so that you don't hit 90 on everything with nothing left at endwalker level to pick up the experience.