I can find no such statement from Cora, and I absolutely don't believe that "coaching them up" is the answer at this level when they all have multiple years of expert, professional coaching under their belts. I will cite three examples of what I am getting at.
The first is Hanley Ramirez slowing up going from first to second base on a liner to RF with two men out--inexcusable, but also the definition of what Hanley sometimes does and no amount of "coaching" will fix that. He's actually better this year than last, but is still Hanley at heart.
The second is Beni, the young guy and surely an ideal candidate for additional coaching. The problem is that he actually is pretty quick on the basepaths, and a manager should want to capitalize on that speed. He is second on the team in stolen bases with 7 and has not been caught stealing. He is tied for 2d in runs scored with JD, and JD has 13 dingers to Beni's 3. He also advances on the basepaths--sometimes taking risks--and gets zero credit for it, but always gets blamed for being an idiot when he makes an out. Rounding 3d and getting picked off the other night was a real mistake, but also an alert play by the third baseman.
The third example is Mookie last night. He is in fact a terrific baserunner. Last night he led off with a double and got doubled off 2b by a soft liner Schoop caught going into CF and quickly threw to Machado covering 2B. The replay shows Mookie taking a lead, but not a big one, and actually heading back toward 2B before Schoop had the ball in his glove. Schoop just made a great play. And that's my point. Good baserunning can never, ever be risk free and this is exacerbated sometimes by the other team's great defensive play.
Given the above three examples, my belief is that "addressing that," if Cora said it, is not as simple as you seem to want to make it. Right now, for example, the Sox rank 4th in MLB in stolen bases and 8th in SB% (79.49%).