Hi!
No, thats not exactly true.
When you order a garrison to sally out, it means they will sally
BUT ONLY if and when an outside friendly force arrive to the region and attacks the besiegers.
So its basically a way to coordinate your units inside a structure with an eventual rescue force so they join strength in evicting the enemy from the region.
If there is no friendly "rescue force" arriving during the turn the sally button will have no effect.
To actually order your garrison to sally out by themselves and attack the besiegers you simply have to drag the stack out of the structure and on the same region with and offensive posture.
Hope it helps
