Compared to the solution diagram, here makes 7 points in reverse sente. The core on the 3 lower lines is 18. In the solution diagram it's 11. The question is, is a Black move sente?

 

If White allows here and Black employs the same tesuji at then 7 more points go to waste. Assuming at , A and C might end up split between them, which grows White loss by a small margin. The score here would be 3-ish.

This makes for a game tree

                 12.5 (5.5)
                /    \
               7 (4) 18
              / \
             3   11

and the move is not sente.