This is no different than the scaling limits of many online media companies, in fact. 83% of the US population is in an urban center. 81% of the US population has access to broadband internet (and not all of them take advantage of that access). Netflix's addressable US market for streaming is smaller than TaskRabbit's for tasks.
If dealing with logistics issues at scale would have been easy, your grocery store would have been able to deliver to your home. Do they ? No -- because its not cost efficient due to the logistics involved.
The purpose of the comparison to Netflix was to emphasize that the addressable market for your product or service need not be "every household in the nation" to be considered "at scale". It's OK for TaskRabbit to never offer all its services in every suburb -- because it's logistically difficult or impossible -- they still have the potential to be a large, profitable company without doing so. I never made a comparison between how easy or difficult it would be to grow.