I'm not sure where the robe actually came from, some kind of early replicator?
Yes, that was the idea -- that transporter technology could allow clothing to be fabricated at will directly onto the body. In The Making of Star Trek: The Motion Picture, there's a chapter called "Spray-and-Wear Clothing" that begins with a discussion of this bit of futurism (before segueing into a discussion of the more conventional methods the film's costuming staff needed to use). A bit of futurism that later productions abandoned.