Now, the picture could be incorrect or stretching the truth, but there was some indication.
Computation (in the context of webpages) indicates JS to me, though they concievably could mean DOM construction. I'm fairly skeptical that transmitting a serialized DOM and unpacking it could be more efficient over mobile network connections that is using our time-honored DOM serialization, HTML.
I do hope I'm wrong, as I can't see how this sort of proxy can be healthy for the web, but I don't think my speculations are out if the realm of reasonable given the information.
Just the CPU thing makes a big difference. There's a noticeable timing lag between loading a page on my iPad vs on my computer. Same wireless network so it's not the connection slowing things down.
I don't know how Amazon is doing it with Silk, but I'm guessing they're seeing enough of a speed up to make it worthwhile.
Similarly, even if JS bytecode was the same size as the JS itself, eliminating the cost of parsing, constructing an AST, (possibly) transforming the AST, creating bytecode, and (possibly) doing an optimization pass on the bytecode would be significant.