I don't get it because it seems arbitrary. There's no reason to think that Splatoon has to launch that early in the systems life. I can sort of understand saying that it should be within the first 3 years or so, but there's no reason they'd need it in the first year of the Switch 2 for "the health of the series".
Splatoon will be healthy whether it gets in the first year or not. Besides, a 2024 launch would probably still have Splatoon coming somewhat early (2025/2026 depending on how long ago they started Splatoon 4's pre-production, which they could have done parallel to post DLC support). Not to say that 2025 isn't a valid prediction, but it's not because of Splatoon ...