The quotes have been taken from the source of this problem.
Hope you have gotten it correct!
Bill: Yes. Probably neither one is sente.
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.