Well...we weren't there so difficult to say..but it's an interseting observation.
ATC are part of the whole team responsible for safety and this particular scenario reflects the excellent and professional relationship that we all enjoy with each other.
The responsibilty ultimately lies with the captain of the departing a/c in this situation. Perhaps the SAAB was able to execute an early to turn to avoid the w/s area? As I said we don't know what he/she could see out of the flight deck window - perhaps by the time they had taxied into position the cell had moved?
As I said, an interesting question and well worthy of comment from others..?