I think the key insight here is that you get a limited number of bits, in design space, to bridge between things that have already been shown to work, and things that have yet to be shown to do so.

That is a very helpful way to put it: "Gall's Law" is the claim that there is this limited number of bits.

Of course, put so clearly, it looks kind of trivial, so I think that we should read Gall as further saying that you can get a reasonable intuitive bound on this limit by just looking at the history of innovation, but that people often propose designs when a little reasonable reflection would have shown them that they are proposing to step far beyond this limit.