After carefully reading the paragraph regarding the tires it does sound like this particular model did indeed run tubeless. Seems I remember back then that tube-type tires were the more prevalent item, unlike today, and this article pointing out that the rims are suitable for tubeless tires (and the bikes certification) kinda gives the distinct impression that the snowflakes are suitable to run without tubes.
I would imagine that the BMW shysters may very well have had something to do with the official corporate line that tubes must be used. It's possible the rims WERE originally designed and specified for tube-only operation and later it was found that they worked fine without them. This not pacifying the bottom-feeders they might have decreed the rim must be ran with tubes in order to absolve BMW from any kind of future liability if one failed. Of course, this is all speculation on my part. I might very well be inclined to try going tubeless on my '81 R100 at the next tire change...