Again, JB has never played the 1-3-1. He has played the guards in tandem alignment to try to make the high post entry more difficult. And that alignment looks like a 1-3-1 initially. But all the rotations after the entry pass are the same as JB's "regular" 2-3. The rotations in a traditional 1-3-1 are VERY different. And there is a reason why you almost never see a 1-3-1 at this level. It's too easy to carve up.
I agree with your observation that we pinch our guards together as a first step to defend the foul line entry pass in the 2-3, but that leaves the weakside wing shot open.
They
did play the 1-3-1 intermittently the last year or two. They would move Buddy to guard the foul line, with Joe on top (not just pinching the foul line from the weakside - Joe was picking up the PG as they came across half court). Jimmy and Cole on the wings. It was very effective.
Some people called it a 1-1-3 at the time, but it wasn't. The forwards didn't play low along the baseline. The forwards played alongside the foul line on either side, with Jesse lower. That's a 1-3-1.
They don't really play the baseline in our 2-3, either. Our "2-3" is actually a 2-2-1 zone, with the center tasked with covering corner jumpers.
We used the 1-3-1 against teams like ND and Pitt last year, as I recall. There were other times, too, but it was mostly when teams were entering the ball at the foul line effectively, the adjustment is like stacking the guards instead of spreading them out.
You are even starting to see that adjustment this year, with Joe dragging down to the foul line when he's on the weak side in the last few games. Symir or Judah will hound the PG out by the 3 point line, and even a few feet above. That's different than our usual 2-3 with the guards pinching the foul line. If only 1 guard "pinches" from the weak side, and the other stays high, when the ball is above the key, it's a 1-3-1. When the ball moves to the wing, it looks more like a traditional 2-3.
If we played a true 2-3, the forwards wouldn't play so high, and you wouldn't ask the center to cover the corner jump shooter, as well as the post and the foul line.
That's been JB's innovation with the zone, to bring the forwards high to cut off passing angles, force turnovers and spring the fast break. But it asks too much of most centers, and it doesn't leave your forwards in great rebounding position.