I suppose adding a new layer would make it somewhat thick, but not necessarily too thick. Any machine coated film is going to have a rather thin coating to begin with, so it shouldn't add much more than several microns.

As for their subbing treatment, having to guess here, I would bet that they coat with an inline corona discharge unit. The pre-subbed substrates on the other hand probably receive a corona treatment, and are then treated with an ultra-thin subbing layer for coating later.