No, it doesn't; existing SFHs can, and have when allowed to, become duplexes, triplexes, and sometimes even quadruplexes without changing square footage at all, with doubling, you can go even further. All it takes is remodeling so that each subdivided unit meets minimum habitability standards (separate access, its own restroom, whatever other facilities are mininally required.)
Well, no, it doesn't assume units are arbitrary, it assumes units are fixed square footage, which they are not. Under most regulatory schemes, there is a practical minimum size or a habitable unit, but a pre-existing area zoned for detached single-family units exclusively is unlikely to be comprised of single-family units that happen to also be the minimum square footage for a habitable unit.