I read that in the clymer manual for a variety of OHC models where it says some valves may be coated. I asked the local head specialist and he said it's crap as he's recdut and lapped  many Postie bikes over the years and has done 3 CT90 heads for me all of which are going fine.
As the CM91 has the same valves as the CT90 I dont think you have to worry. Maybe some of the other more unusual models the manual covers had these special coatings.