My point was that there are four combinations, and "both, either, or neither" suggests that all four can happen. But actually, only three out of four can happe

My point was that there are four combinations, and "both, either, or neither" suggests that all four can happen. But actually, only three out of four can happen. It can be both a statement and an expression. Yes. It can be neither a statement nor an expression. Yes. It can be a statement but not an expression. Yes. But the case that it is an expression but not a statement: this case never happens.