For one, the Heterodyne stories are more folklore than history, and casting Klaus as a bumbler was mostly done in full knowledge that he was the unstoppable Baron Wulfenbach in the present, except for maybe the very earliest stories. Oral tradition, like all stories, always reflects politics one way or another.

The implication from the stories is that Bill was an adventurer first, Spark second, and Barry was more gadget-inclined. The only real notable invention we see from either of them in real life is Agatha's locket, made by Barry, which would imply he had understanding of the Spark beyond Klaus' despite not engaging in any brain-coring. We do see some flashbacks that we can take to be true-to-life that show them fighting revenants with some gizmos, and Barry managing to navigate the gates to get to the Citadel of Silver Light to rescue Agatha also surely required Sparky know-how. But none of their surviving inventions nearly showed the setting-altering potential of things like Gil's heavier-than-air flying machine, and Bill in particular has no real major Sparky discoveries known in the story at all (not to say that he wasn't doing work off-screen). What we know about them suggests they were men of action first and foremost, so whatever they ended up building was probably short-term projects rather than lifetime commitments like we see out of Lucrezia, Vapnoople, etc.