That doesn't make any sense. A fighting game, especially a 2D one, doesn't have overly complicated physics, and rollback is about split seconds, so easily doable.
Now the tweet talks about an *emulated* game. In this non-native case the whole emulation has to be done, including the (in case of rollback) pointless rendering. That is much more work needed.
Rollback is also CPU intensive. The simpler things are, the easier it is. But the more stuff you have going, the more things need to be taken into account. It's not just physics: you have to consider sounds, the various command inputs if different players for example: all info that is being registered needs to be accounted for rollback to work. Depending on the degree to which rollback is used, more frames can be kept and adjusted.
The Nikelodeon fighter supposedly only has rollback for 1v1 on the Switch. 2v2 doesn't support it:
Clarification on rollback netcode for the game, from Rep on Discord : AllStarBrawl (reddit.com)
Additionally, 'emulation' is most likely being used to account for the game having to emulate what happened before, and then to emulate what the outcome would be (hence rollback)
(9) Mike Mika on Twitter: "@jotamide @Pond3r @necrosofty @VOLGARR @DigitalEclipse It does have rollback. Switch suffers from how many simulations you can do for rollback due to CPU speed." / Twitter
Why doesn’t every game have rollback net code? : Fighters (reddit.com)
If the Switch is unable to support the same level of rollback being seen in the other versions, then that itself will be a problem for cross-play. So, they either 'ruin' the online experience for everyone because Switch users can do cross-play, or they close-off the Switch version as it's own thing. Neither is ideal situation. If anything, they'll just wait for a stronger Switch to release.