You aren't really feeding it one array, you are ANDing 3 arrays. This

=SUMPRODUCT((ISNUMBER(Y!$1:$3))*(Y!$1:$3<=Z!A1))
is the same as this

=SUMPRODUCT(--(ISNUMBER(Y!$1:$3)),--(Y!$1:$3<=Z!A1))
but you cannot use the latter form when you introduce the 3rd condition, as it is a different shape than the other two.

And yes, you could use SUM but you would have to array enter it. But that is true of most if not all SUMPRODUCT solutions.