Before the season, the Sox called Devers long term as a 1b/DH and tried to extend him with an AAV similar to the deal Matt Olson received from Atlanta. And long term, Devers probably is a 1b/DH.
In the last 3 years, his OPS+ ranks third among active third basemen, behind only a Jose Ramirez and Manny Machado, and just ahead of Austin Riley and Nolan Arenado. As a first baseman in that same timeframe, Devers would have been tied for fifth in MLB among qualified hitters, trailing Goldschmidt, Guerrero Jr., Freeman, and Alonso, and tied with Matt Olson. (Maybe the Sox use of him as a contract comp wasn’t so random.)
If the Sox are valuing him the same way as other teams that have extended their players, this goes nowhere fast. And if Devers thinks he’s a $400mill player, that will get him traded even faster.