It's Dalbec a plus defender at his original position, third?
I can't imagine Cora is legitimately trying to convince anyone Dalbec is a plus defensive shortstop or even at first.
Dalbec, early in his minor league career looked like he might become very good at 3b. There were endless debates, once upon a time about how Dalbec may be better suited for 3b whereas Devers might have to move.
But as he's aged into his mid 20's he's lost a lot of athleticism. That isn't a dig at him, it's not like he's shown up out of shape or anything like that, but he's just reportedly regressed in terms of mobility. At present time he's not really an option there for a full time position. He was also really bad at 1B last season.
I think Dalbec playing different positions this Spring isn't an endorsement of his ability, defensively. Rather, I think it's a reflection that they still like enough about his offensive upside that they'd like him to get reps all over the field to serve as an emergency option on the higher end of the defensive spectrum. I also think they probably want to see him get regular reps at 1B, still to spell Casas and maybe play a little OF, as well.
The guy can hit bombs and when he's right, he can also take a walk. He's at least a 70 raw power guy. That's why they're doing this. Not because they think he can field. Ultimately, if he's able to bounce back, his best internal path to an everyday role is likely to be to replacement of Turner after his contract expires.