I think most of NLG worked on Battle League. It just doesn't look like that because there were a lot of contractors on Luigi's Mansion 3 that had already left at that point.
The only real indication that they've been working on another project even before Battle League is that
Bryce Holliday and a handful of other designers were only credited under "Additional Work" despite apparently still working there.
Akiya Sakamoto from EPD 6 wasn't credited either, despite working on every other NLG title published by Nintendo.
I guess it isn't dissimilar to Luigi's Mansion 3 being in pre-production since the Wii U days. Whatever they're working on now probably didn't begin full production until the latter half of last year.