Hi
I think you need to also coerce your Lefts from Text values to Numbers and, your second part I think the test should be = and not <>
Code:
=SUMPRODUCT(--(--LEFT([workbook.xlsx]Sheet!$A1:INDEX([workbook.xlsx]Sheet!$A:$A, MATCH("zzz", [workbook.xlsx]Sheet!$A:$A)), LEN(C1))=C1),
--(--LEFT([workbook.xlsx]Sheet!$A1:INDEX([workbook.xlsx]Sheet!$A:$A, MATCH("zzz", [workbook.xlsx]Sheet!$A:$A)), LEN(D1))D1))