I don't remember it glowing because of overgrow. All I remember is it glowing because it ate it's Energy Ball!!
Hidden Abilities weren't a thing until Gen V, so Overgrow can be safely assumed.I remember him swallowing Energy Ball a couple times when he was a Grotle and I guess that's where the glow came from?
But Torterra definitely never had Overgrow because it was never mentioned or confirmed in the show.