I think the earlier airheads have greater claim to that. A mate has over 400,000 miles on his R75/5 - and I bet it's seen more dirt in that time than 99.9% of the current crop of adventure touring bikes will ever see. Even my old R90S, which I still have and would love to have up here, saw dirt pretty much every time I rode it.
Hans Muth, the designer of the R80G/S did not mention the word "adventure" once in his presentation(s) about how the G/S came about. Closest he came was the word "enduro."
As Ian mentioned above, motorcyclists had been doing what now is called adventure motorcycling long before the BMW R80G/S came into being. Guys like Danny Liska traveled from Alaska to the bottom of South America and through Africa in the 1960's on a BMW R60....and then there was Carl Stearns Clancy who circled the globe in 1912-1913 on a Henderson Motorcycle (See the book MOTORCYCLE ADVETURER).
Any motorcycle is an "adventure". Not just the current "fad" of so-called adventure bikes.
I started out riding an almost standard (DIY G45 replica mufflers and a Dunlop universal on the front) Matchless G9 500cc in 1960 - on seal and plenty of gravel roads. (Not exactly dirt but close to it when wet!). For the last 12 years I have been riding, among other bikes, a standard (mundane) Kawasaki ER-5 that most of you would not even look at twice. But we have had many adventures over 78,000 km, with the original chain & sprockets and original front brake pads. A replacement battery, cleaned fuel valve and repaired instrument bracket are the only repairs/replacements other than new handlebars because it fell over during the Christchurch earthquake(s). I turn 80 years tomorrow (if I make it through tonight). Planning a 1600km round trip next week as my brother turned 75 years on the 20th. April and we plan to celebrate. He still has his 1951(?) ES-2 Norton (since 1970's). It starts with one "kick" and idles faultlessly. And, boy, has that been. an adventure. Enjoy your motorcycling: no matter what your bike is - like life, it is an adventure. May the Tail Wind Spirit be with you.