Not necessarily,
I just sold a Model 15 with a W on rear of sheath. Not waxed but made of Latigo leather. Was told when I received sheath it was made of harness leather. Very thick. Johnson sheath. 1990.
When I ordered the knife I told Randall I wanted a waterproof sheath and that's what I received.
This is the first I have ever heard of anything like this. Historically, W stands for waxed, not "waterproof".
Never have heard of Latigo leather either as being something different than what Johnson used everyday. The inference you make is this type of leather has waterproof qualities?
Who exactly told you this? Johnny Johnson? Someone from the shop?