Instantaneous sum of the common mode currents at B (you can give it an
overall minus sign if you want, but it's right for common mode):

Current flowing toward B from X + Current flowing toward B from Y -
current flowing out from B via coax shield - current flowing out from B
via coax center conductor.

All the common mode current on the coax will flow on the outside. So
current flowing out from B via coax center conductor is zero.

So current flowing on the coax braid = Current flowing from X to B +
Current flowing from Y to B.

Points A and B are one point except for the reactance of the short
connecting them.

Dan