Sheldon was alive while 650B or 27.5 was around. They have been around at least since the 1940s on touring, randonneur and utility bikes bikes especially in France (since the 650a, b and c are all French sizes) and in terms of mountain bikes Tom Ritchey and others were making off road 650b bikes in the 80s. They used Nokian tires imported by Gary Fisher on Wolber Super Champion rims.

This is not a new size at all it is quite old school. Heck people probably were doing some form of mountain biking on 650b tires for a long time before even Ritchey and Fisher and others were up to it back in the 80s and before it became the brand new tire size that had never existed before in the 2000s. My guess is once the safety bicycle came out people were probably doing some sketchy stuff deeper off road pretty soon after though the tire size may not have been 650b.

Sure I will say that modern bikes are certainly easier off road these days and the tires have greatly improved but this is not new.