I don't consider a 60/40 to be a rain coat either. If there is a steady rain outside, I always pass up my Cameraman and 60/40 for full goretex (with taped seams). Sure, a 60/40 or Cameraman would be fine in a light rain, but neither would last long in a steady rain.
I suspect the reason why the Mackintosh top of a Cameraman does not have taped seams like a full Mackintosh coat is because they are intended for different functions- the Mac is a raincoat, and the Cameraman is for snow:
"Inspired by a jacket worn by Wilfrid Noyce on an Everest expedition, the Cameraman jacket has an upper made of Mackintosh,
and a lower made of Harris Tweed, to keep the wearer warm in extreme conditions. It features specially made metal clips, and waxed cord in the hood and hem."
It generally doesn't rain on Everest.