Its exactly as he says. Every zone has a specific level range attached and that never changes. The only exceptions are special static spawn Pokemon that can be dozens of levels higher than anything around it, but they are incredibly obvious when you spot them so you don't accidentally kill yourself.

In fact, most advice people give going in is to rush one of your 3 main quests, because its the one that locks your movement ability behind it. So once you know that you are much more incentivized to bumrush bosses 10+ levels above you so that you can unlock swimming, sprinting, and gliding. I think I did the level 57~ one at like 45 just to unlock my last one.

Its helped by that being the quest with the most emotional resonance and writing to it, where you are far more likely to be invested than the more generic "gym challenge" and the more comedic "evil team" quest.