No doubt that H-D-H were a terrific songwriting team and possibly the greatest for Motown.
However the Motown story, like that of popular music in general, is far from that simple. Eddie Holland was also a singer and had a few hits before quitting performance due to chronic stage fright.
Here's a summary of the many others who were big songwriting contributors to Motown:
https://www.udiscovermusic.com/stories/motown-sound-producers-and-songwriters/
I don't think the link above mentions R. Dean Taylor, who, though probably better known here for hits like ' Gott