It really doesn't make any sense. There are 4 seasons in a year - 4 season passes can be released within a year. Expansion pass makes way more sense.You can actually can buy Season pass now for original Mario + Rabbits even tho the game came out in 2017.
Some called expansion pass and some called season pass, but MP games are only ones with expiration date.
If they just said DLC will come out in 3 months I would be fine with it, but wording means a lot in this business and it should be obvious to anyone by now.