You're right that there is not an option to select "studio" under the type of listing, you just list whether it is a guesthouse, condo, etc. But when you choose the number of bedrooms, if you list "0" bedrooms and then the number of beds, it will call your listing a studio instead of a 1 bedroom unit regardless of the number of beds you select.
Guests can search for the number of beds, or the number of bedrooms. When I travel with my family, I always want at least a 1 bedroom so that my husband and I have privacy.
I understand why people choose 1 bedroom instead of studio if everyone else id doing it, because correctly listing as a studio puts them at a disadvantage compared to all of the other studios listed as 1 bedrooms. Instead, 1 bedroom units are at a disadvantage. I think there should be a price differential between studios and 1 bedroom units, but if I list my 1 bedroom more expensive than other studios that come up in the 1 BR search, it looks like I'm listing my 1 bedroom unit too high.