You could argue that Verizon cell phone game was a spin-off.
That said, none of the other games released could be considered "spin-offs", since they all tell an integral part of a larger over-arching story. It's like trying to say anything that isn't Avengers-branded is an MCU spin-off.
The main 3 games tell the story of the main character Sora in his battle against the main villain. The side stories are well integrated into the main story, but they are still side stories. Even the article I cited used Kingdom Hearts as an example of a series with tons of spin-offs.
And no, your MCU example doesn't work. MCU is a universe. If Avengers were the main series, then yes, the Iron man movies, Captain America movies, etc would all be spinoffs of Avengers. They all take place within the MCU. If you don't consider Avengers the main series, but just a series that takes place within the MCU, then all the series are just IPs with stories that interrelate. Harry Potter would be another interesting example here. The first 8 movies were a standalone series, and then the "Fantastic Beasts" series was made, which is a spinoff to the first series, even though it contains story details related to events in the first series.
What I don't get is why everyone is so damn upset by this. Who cares how the games are classified? That doesn't decrease their legitimacy, or their fun.