I agree, and honestly, I have no idea when he will be "deemed" ready.
IMO, his bat is ready to be tested at the ML level, but part of that decision involves evaluating a player's confidence, ability to handle set-backs, ability to make adjustments on the fly, and other aspects none of us may be aware of.
His defense almost certainly needs more time, but they might think it's not a big enough drawback that would drag down the plus his bat projects to be. Again, they might also be thinking about how he might react, if he makes a couple costly errors or gets roasted by the media for "taking wrong routes to the ball" or whatever.
I'm not sure any of know about his emotional and psychological make-up- not that the Sox know it all, either, but that is one area that may or may not be a big concern.
I remember seeing Devers come up and thinking, "nothing phases this kid," and I mean that in a good way. He could make 3 errors, and still hit the winning HR in the 9th. I think that was one reason they had no problem fast-tracking him to the bigs- that and the fact that throughout his whole professional minor league career, he was the youngest player at every level.