Honestly, they don't need to explain it. Think about TAV. They had been training with Master Eraqus for years, and should have had at least mid-level magic and combat skills but instead they are all at a very basic level gameplay-wise. Aqua is even made Master with her very puny and weak in-game skills.
Instead we should just suspend disbelief and start KH3 with a lvl 1 Sora because it's just a game mechanic.
Now, what I would have wanted is the game to open up with us doing the tutorial in one of four ways:
1) Start the game as Kairi in training. This is a very basic choice that most fans hope to see because we all are rooting for Kairi to become a strong fighter rather than a damsel in distress & convenient plot device. We would play her for 3ish hours, get her up to level 5-15 with some very basic skills, then switch to Sora who would take on the same level as Kairi and have his skills & commands that equate to that level. Also he'd already have all of the basic level magic available to him because I would believe that in KH3, we'd start getting more -za spells which would mean only having to earn -ra, -ga, and -za spells in that order.
The problem with this is that Kairi's play style will probably be very different from Sora's play style. It'd be different if this was a multi-scenario/multi-character game, which is still a possibility since we know absolutely nothing about KH3. It isn't a very serious issue, though.
2) Begin the game where we left off in KH3D, which was in the Realm of Sleep with Sora. Somehow, Sora's Dream Eater friends would guide Sora to a world that is deep in sleep (this would account for why Sora is "so late" by Donald and Goofy's terms) and this would be the Land of Departure. The thing about this would be that the LoD isn't dreaming of the last events that led to its ruin and change into Castle Oblivion, but rather an event farther in the past that the sleeping world wishes to show Sora.
We'd start the tutorial by playing as a young Eraqus training to become a Keyblade Master. It would specifically hone in on the events surrounding Xehanort's discovery of the Foreteller's Script. This would be a fine way to impart this knowledge to Sora who is not playing these events but watching as an outsider, and thus introduce one of the mysteries of the game to us. Also at the end of the dream, maybe it could be Master Eraqus who speaks to Sora because his heart is sleeping and somehow able to reach Sora IDK JUST AN IDEA
Of course Yasue said that the scene with Eraqus and Xehanort discussing the legend opens the game up so who knows.
3) Same scenario as above, but as Young Xehanort. Personally I would be against this because it should stay a mystery to us until the middle of the game how Xehanort came to find out about this knowledge.
4) Start the god damn game as Sora. There is no need to deal with tutorial stuff. Give Sora access to all basic level magic, a few puny abilities, and call it a day.
Well technically Sora never actually time traveled
Sora and Riku did time travel. Yen Sid used some random loophole to send them back in time just before the night of the storm. They steal their own raft, and then after fighting Ursula they open the pathway to the Realm of Sleep.
