The 95% CI on a true .32 probability in 607 trials is (give or take) +/- 038 ... so this is probably a lower true rate but let's not get too excited just yet. Not that it's surprising nor is it something anybody can really take advantage of that I can see.

hitters who barrel fouls at high rates also typically barrel their balls in play at high rates

How is this anything other than blindingly obvious?

I guess it's nice that somebody took the time to check.

#2: Also clearly nobody does it at a "high rate" ... this study consists of about 50,600 foul balls of which 600 were barreled. So some guys do it at a "higher" rate. Looks like the average batter would have about 450 foul balls in 650 PA (more than I would have guessed). If we borrow that foul barrel rate, they'd hit about 5 per year. 8 would be "a lot", 10 at the top of the random range (give or take).

Steve Stone always warned about those foul balls that were rockets straight back to the screen, suggested the pitcher not throw him that pitch again in this plate appearance. (He was usually right.) I don't know if those count as bullets here or not. Those happen when the guy's timing and swing speed are perfect but he nearly missed the ball entirely so the "launch angle" is terrible. Foul bullets would be ones where the swing plane and bat speed are right on but the timing is off a bit. No doubt Stone would (and probably did) say the same thing about those.

The 2nd paragraph of the excerpt suggests the difference is primarily Ks becoming BBs ... is that established further in the article?

hitters who barrel fouls at high rates also typically barrel their balls in play at high rates

How is this anything other than blindingly obvious?

This just in: Guys who hit ballz hard is gud at base ball.

#2: Also clearly nobody does it at a "high rate"

I don't know. Gary Sheffield seemed to do it at least once per plate appearance.