I don’t know if it will still over-level pokemon, but it has been changed compared to past games. There’s footage of gaining EXP ranging from high level pokemon to low level ones (in one party). The high level ones barely gained experience while the lower ones gained a lot. (image at the bottom)
So over-levelling, at least compared to the rest of your party, doesn’t seem like it’ll be a problem. Whether you’ll still be over-leveled compared to your story progression, who knows. It might make it easier for them to manage difficulty. Before you might train up a lower level pokemon you got part way through and ended up over-levelling another. But with how this is adjusted, it doesn’t look like that’ll really happen (at least not to the extent it normally does). So they might have a better idea of your team’s overall strength at each point in the game because it’ll be consistent.
That being said, I hope trainers/gym leaders/elite four have more pokemon in their teams. I want to see more enemies with 4-6 pokemon. And not just the joke ones like “all magikarp/bug/low level pokemon” where you steamroll through them even without grinding.