Yes, it is possible (even probable) that some of the natural fibers can be found in deniers finer than stated (as I don't have my old reference books available at the moment - I had to rely on what I could find on the net). Silk is another story altogether (haven't seen many silk detailing towels on the net).
Cellusoics can be made spun/extruded as a microfiber, but these would still be considered man-made fibers (although not synthetic), as they do not appear in nature in this form (they typically start as woold pulp).
I'm not aware of any naturally occurring microfiber cotton - not to say that it doesn't exist . . . I just haven't seen it.
So yes, some of the information may be outdated (or even wrong). I tried to do some quick research via NCSU, Google, Cotton Inc, etc. However, even with any errors (which I sincerely regret) the basic jist remains true. Microfiber fabrics, if properly constructed, are perfectly suitable (and perform better than most natural fibers) for the purposes of automobile detailing.
Not looking for a flame-war, only trying to contribute a little bit to this great forum.
:wavey