It has always been an inability to square up fastballs in the zone that has been his primary weakness anyway, IMO.
I disagree, I think his problem has been putting himself in a bad position to be aggressive by chasing junk.
Not to be flippant (for once in my life), but:
The recent video posted was a little hopeful - there was one very close checked swing on a breaking pitch in the dirt, that maybe in past years would have been a full swing and miss. I don't know whether the clout on the curveball was on a hanging pitch - there's a tendency to say "that was a hanger" if any breaking pitch goes for a base hit, and I'm not more nuanced than that. I presume that some of the HR this spring have been on fastballs, and like Chief I've been bothered in the past by clean misses on pitches down the middle. All in all, the video shown here (and the overall stats) hold out some hope on the multiple fronts where improvements were needed, but it's far from conclusive - the balls that went for base hits could easily have been slightly different and have gone for routine outs - his BABIP is probably out of sight right now. I'm optimistic but not convinced this is more than a blip.