It's hard to say, because Harada once tweeted something after Sora's announcement that implied the people who worked on FP2 would be moved to Tekken 8, and the gap between SSBU patches:
This page documents the update history of Super Smash Bros. Ultimate. All additions, fixes and changes are grouped together by the update they were introduced, including unlisted changes found by the community. Some updates have corresponding...
www.ssbwiki.com
lines up with when the majority of Tekken 8's development would occur, not to mention that if Namco's Studio S is supposed to be "Studio Smash", it would be weird for them to form that company midway through a new Smash's development.
But on the other hand I've looked through the base SSBU credits once on MobyGames, and its developers are mostly Tales/Ace Combat/Soulcalibur devs, not Tekken ones.