Bowie's 'dress' actually looks more like an overlong frock coat or priest cassock than a dress.
Yeah, I can see the similarity in the design. But he did call them "dresses". They were designed by a guy he was dating around that time (c.1970-71) named Fred Burrett (sometimes known as Burretti). Burretti was a fashion designer and was developing a line of men's dresses at the time for his shop in London. I guess they were specifically designed to account for a man's flat chest. The one in the picture is, I think, the same one from the cover of
The Man Who Sold the World, so is the most famous. But, Bowie had a lot of them, since he wore outfits like that fon stage during the period.
Burretti also designed some of the very, very early Ziggy Stardust costumes (early 1972). Most of the more famous Ziggy costumes were designed a bit later by a Japanese designer named Kansai Yamamoto.