I suspect the rule against exponential growth is neither definable nor enforceable, but it kept the book from turning into a series of dreary territorial fights.

Yes, I take that as very similar to Vinge's Zones of Thought - it's almost moronic if you try to take it as any sort of plausible prediction or resolution to the Great Silence, but both reader and author agree to suspend disbelief on that premise in order to keep human-level stories possible.

(At least, I really hope Egan doesn't seriously think that there would be any gentleman's agreement against exponential growth, because his particular multiverse makes the problem of the Great Silence exponentially worse than it already is.)