Yes, he was paid the bonus up front, but up front bonuses and deferrals are all factored into the Average Annual Value of 30.4 million (or 365/12), which is clearly the fairest measure of what he is actually making.
If we only use what the player is actually being paid, Ohtani is only making 2 million for 2024. But his AAV is 46 million because of the deferred money.
Also, to get really technical, in 2020 players only received 40% of their salary because of COVID, so that year Betts would have been paid about 11 million instead of 27 million.