What was lacking was the ability for concept thinking and language.
How so? What evidence do we have that the exocomps were not talking with each other, using a complex and expressive language?
If anything, we could argue that our heroes were not sentient, because clearly they didn't speak exocomp.
Machines can't naturally "evolve" the way organic beings can.
Why not? It's the organic beings that cannot evolve much (from infant to adult, yes, but that's not really all that remarkable) - but a machine like this could and would be built with the ability to improve itself within a single generation, and there'd be no point in building in limitations on exactly how
it would improve itself (or else it would have been built to the improved specs already from the get-go). So unpredictable results would be guaranteed by design.