So what you are saying is that if Bou had spent all that bicycle training time on a moto, he would not be where he is today?
Maybe that's right. I'm not arguing against it, but is it obviously and definitely the case?
I would think that the skill which transfers the best is the ability to maintain a dedicated and methodical practice regime.
Apply that to either/any discipline and you will get results. Another way of saying that a person who can teach themselves to "hop" on a bicycle could probably also have learnt on a motorcycle.
I don't find my 300 pro intimidating at all as a first trials bike. It seems fine but I have read enough from experienced riders to suspect I may actually do a bit better in sections on a smaller bike. Anyway I guess if we're happy with our bikes it doesn't get a lot better than that...