Did some IMDB investigating...
Heidi was an Exec Producer on the show from it's start in 2004-2017 (not every single episode but almost all of them) and Karlie is an EP now in the 2.0 version. So they are and were more than just hosts.
EP can mean a lot of different things, levels of involvement (sometimes it's name cred, other times it's $, most often I think it means they are actually making the show come to be) so I'm told by people in the biz, but it is definitely more than the hired help.
To compare, in the new reboot, Nina is a consulting producer and Brandon is an associate producer. Nina isn't listed with any kind of producer credit in the 2004-2017 shows.
Oh, I didn't think they were just using the design, I think she actually wore the physical item made by the designer so it had to fit her. Thus all the models had very close measurements to hers.
I would hope so too! For a zero-waste challenge theme that would be super tacky to remake it out of other material.