Darn has the "room for more upside" narrative going for him that none of the other catchers below Grandal do, which is why he was considered the best of the rest. I think the thought process is something along the lines of:
1. He could be great (3+ fWAR) if he stayed healthy.
2. The Mets are probably the worst organization in the league at keeping players healthy.
3. He stayed healthy the moment he left the Mets.
4. He is more likely to stay healthy with the Braves than with the Mets.
I think I would have preferred Castro as a clean platoon partner with Flowers, but it's hard to criticize AA making the upside play with such an affordable contract. If Flownaud posts 4+ fWAR for $12M of the 2020 payroll, folks are going to be tripping over themselves to talk about how smart AA is.