defense was man-to-man. Nobody had the TE. That's not a good man-to-man defense if you only assign a man to 4 of the 5 guys.
A Zone does it by covering zones. The TE is naturally accounted for in a zone defense since the entire field has a defender assigned to it. Not that a zone doesn't have seams, gaps, holes and other issues that can be abused, but it at least has everything accounted for at the snap.
But I've seen plenty of TEs free release at the line and be wide open the entire route, as in nobody was ever accounting for them on defense. I've seen plenty of RBs just jog out into the flat and there is nobody there on defense to account for them.