Are you using some different definition of "preserved" to my dictionary?
Preserved: something kept in its original state.
The SMR seems to fit that definition admirably, in fact I struggle to think of any other heritage railway that is better preserved (the Great Orme tramway, perhaps?). It is the original railway, carrying out its original function, on its original route, using much of its original infrastructure. You can still ride up to the top of Snowdon powered by the exactly same steam loco as the Victorians did.
It would seem to be far better preserved than either the Talyllyn or Bluebell, for example, neither of which now much resemble what was there pre-preservation. The DSR is a more debatable case, as I understand some of the stock may have run on the line originally, and the line was mainly built for tourists, but not run in the way it is currently being operated.