Let's see what their role was in the FH series:
Producer in Forza Horizon 2, 3, 4 and 5.
Chief Engineer in Forza Horizon 1, 2, 3, 4 and 5.
Technical Artist in FH 1 and 2, lead technical artist in FH3, Technical Artist Director in FH4 and FH5.
Audio designer in FH2 and 3, senior audio designer in FH4, Lead Audio Designer in FH5.
Associate art director in FH1, art director in FH2, FH3, FH4, FH5 (left to work as the art director of Vampire: The Masquerade - Bloodhunt during development, didn't complete FH5)
Game designer in FH2 and FH3, principal game designer in FH4, creative director in FH5. Also writer in FH3, FH4, FH5.
See above, I listed where they did work before. They are basically the key people of the Forza Horizon series team, who leave PlayGround to create their own studio (different than the Fable one). I understand they could have moved some FH staff to the Fable team, but I highly doubt that they'd move most of their top staff to the Fable team.
I assume many other talented will remain there who could replace them, and if not they'll hire other talented people capable of doing that job.
I thought the same, but seeing they are working on an open world game they left because are tired of making racing games during so many years.
Also, if Sony is interested on a Gran Turismo arcade game, Firesprite has a ton of key former staff from Evolution, Bizarre Creations and Liverpool Studio.
So I wouldn't bet on it.