From the snippet provided, you were trying to use "available float" in the "critical path". The critical path by definition has zero float. so that answer is by definition is incorrect. As you suggest, the PMBOK (pg. 211) agrees because "the project’s critical path is not changed", that is we are making sure resources are not say more than 80% optimized. Even though the critical path isn't changed, it doesn't use "available float".
Read the question carefully, the PrepCast explanation is correct.