That's not
quite true: there are a very small number of things you can miss, but none of them are that important. Some Potions, a weak piece of armour you could buy anyways, that sort of thing. All of them are in the prologue.
The most notable missable is this, and it barely counts:
If you beat Setzer during the Struggle tournament in the Prologue, you get the Champion Belt (+20% Fire, Blizzard and Thunder resistance).
If you lose to Setzer during the Struggle Tournament in the prologue, you get the Medal (+1 Strength).
You can't get both. Neither is great in the long term. The Medal is definitely worse.
The Kingdom Hearts series is pretty good at avoiding missables in general! Let's think. There are some pointless chests on Destiny Islands in KH1. There's the infamous Red Trinity in the original KH1. Bonus Bosses in KH1 and DDD can lock you away from chests temporarily. Nothing in CoM, nothing in Days, nothing to my knowledge in coded or DDD. Birth By Sleep has a few pointless missables, though if you consider the Final Chapter to be locking you out of the previous chapters, that's a pretty glaring flaw.
Just a lot of "kind of, sort of" situations!