Instead I read it as something like "some unreasonable percentage of an agent's actions are random"
This is in fact the intended reading, sorry for ambiguity. Will edit. But note that there are probably very few situations where exploring via actual randomness is best; there will almost always be some type of exploration which is more favourable. So I don't think this helps.
We care about utility-maximizers because they're doing their backwards assignment, using their predictions of the future to guide their present actions to try to shift the future to be more like what they want it to be.
To be pedantic: we care about "consequence-desirability-maximisers" (or in Rohin's terminology, goal-directed agents) because they do backwards assignment. But I think the pedantry is important, because people substitute utility-maximisers for goal-directed agents, and then reason about those agents by thinking about utility functions, and that just seems incorrect.
And so if I read the original post as "the further a robot's behavior is from optimal, the less likely it is to demonstrate convergent instrumental goals"
What do you mean by optimal here? The robot's observed behaviour will be optimal for some utility function, no matter how long you run it.