Can "if not that" mean "if that is not possible"?
1-I want ice-cream. If not that, then a piece of cake.

Academic
Can "if not that" mean "if that is not possible"?
1-I want ice-cream. If not that, then a piece of cake.
Yes.
I agree, but is it the same in AE I wonder?
I'm not a teacher, so please consider any advice I give in that context.