This makes no sense on any level. First of all, free agency doesn't open until after the draft. So this is already dead in the water before it even gets started.
If they want Rozier, they can just sign him. He'll likely come cheap after the year he just had, and few teams will be in the market for him. Most teams already have a starting PG who is better than Rozier, and he just spent all year proving that he's incapable of helping a team off the bench. Phoenix will be on a very short list of suitors, and they have no incentive to pursue a S&T with Boston.
Also, why would they lose the 6th pick for Rozier when they could instead use it to draft a guard who is better than Rozier? Darius Garland and Coby White both project to be available.
Lastly, I don't think Tyler Johnson isn't a negative asset. He'll be a $20M expiring contract, which are frequently very valuable to teams looking for a cap reset button.
Phoenix is dumb, but they aren't this dumb.