I'd disagree with that pretty much completely. FF9 has a whimsical light-heartedness through most of it that the previous two FF games didn't, on purpose. Having replayed 9 earlier this year I can't even say the lore or the world is all that deep either. I felt it was very surface level to the point where Kuja didn't make much sense as a villain, well he did but his build up was garbage he felt like he was the bad guy because they needed a mascot level bad guy. The military shit feels like it comes out of nowhere because they needed a reason to explain away the bad things and the clone angle.
But unlike 7 there isn't a lot of origin in the game. Like there is stuff in FF7 that happens 1000's of years before the game even begins and you dig through a lot of history with Shinra characters, and even characters you never really met in game like Lucretcia and Gast.
Not that FF games need lore really, that's not really useful unless they are expanding on the original plot.
Whimsical light-heartedness? Did you miss the part where an entire kingdom was massacre almost down to the last man, woman and children (which you visit and literally have to walk over the corpses) and then the place where the refuge ran to was nuked to the ground killing everyone inside? On top of that, the other two major settlements in the world were directly attacked and you actually visit them post destruction. Rough estimation, probably half the world population died between the start and the end of the game.
There's also the part where a mom tried to kill her own daughter in a mad quest for power and used sentient creature made literally only to kill other people. Or a tribe being slowly driven to extinction until only a little girl remain. 9 is freaking dark.
As far as lore, both are pretty similar, the origin background in 9 is about the same as 7, with stuff that happened 1000's of years in the past, with Terra dying and then trying to invade Gaia as a last ditch plan. All this is very similar to 7 (the otherworld invader: Genova/Terra, that ancient magic tribe: the ancient/the summoner tribe). I'd argue the Terra aspect is more properly fleshed out than Genova ever was, since you go trough multiple location dedicated to fleshing out their plan (lifa tree, oilvert, ipsen castle, element shrine).