Im guessing they cant because the book talks about retreats only in the "general combat" section; says nothing about sbr planes retreating, but i am not sure.
For "strategic" bombing, a bomber stays over the target for one round. That is, it survives, or not, depending on the roll of the die, and the damage it does is also determined by the roll of the die. After these two determinations, the bomber returns to its home square (if it survives).
For regular bombing, the bomber continues fighting as along as there are land forces making an attack. In this case, the attacker withdraws the bomber when either 1) the land forces retreat or 2) the land forces win and occupy the space. The bomber cannot land on the occupied space in the same turn (the airfields have been destroyed.)
You are 100% correct. Retreating is part of general combat, Strategic bombing is a special type of attack that has no concept of retreating.
AA units only get one shot at the inbound bombers, so unless you changed your mind about the bombers actually carrying out the attack, retreating has no meaning with respect to strategic bombing.