This is not always true. Plenty of founders who spend time working instead of networking end up with a sparse network and very few people even aware of their success.
Less true if you are the kind of startup which could get talent acquired in the first place.
In the bay area at least, it would be really hard to be working on something worthwhile and remain totally sequestered from a network. The bigger problem is not getting caught up in it to the exclusion of spending time working.
I'm sure it is different outside the bay area, and is absolutely different outside the USA.
