Sarcs
Paragraph (a) is only one of two, cumulative, criteria for disallowance to take effect under the provision you quoted.
Paragraph (a): notice of disallowance given in time – tick.
Paragraph (b): 15 sitting days have elapsed after the notice of disallowance was given – cross.
In any event, 42(3) is probably going to be an important provision in this case.
If the House of Reps is dissolved before the motion is voted on or 15 sitting days of the Senate elapses, the business before the Senate, including notices of motion, terminates. It’s therefore almost certain Senator X’s motion of disallowance will be terminated - i.e. no longer have any effect.
However, an effect of 42(3) is that the CAO will be taken to have been laid before the Senate on the first day the Senate sits after House of Reps was dissolved. At that point, Senator X or someone else will (again) have 15 sitting days in which to move a motion of disallowance, and there will (again) need to be 15 sitting days after that motion, or resolution to support it, before the CAO will be disallowed.