I think it's as simple as, this guy was drafted by Dombrowski, and wasn't by Bloom.
Plenty of scenarios in which he's useful, none of which involve handing him a starting spot. If they don't sign a SS he'd join Kingery and Segura to share time at both positions, if they do he could still make the team off the bench if Kingery is definitely a starting 2B or platoon CF. And who knows what 2022 will bring.
Certainly Holder and Chatham won't both be on the 2021 MLB team. If Chatham wins a spot Holder's gone; if Holder wins a spot Chatham can still go to Allentown.
Ideally - assuming Didi comes back or they sign some other SS - they still move Segura so you're not paying $15 million for a bench player.