Disagree with your assessment the founders are not customers. In the sense they paid for their cars, I believe they qualify as Tesla's first customers. Where they fall short in being a typical customer is the customer/manufacturer relationship they have with Tesla is very skewed toward Tesla. As pointed out in another thread, they are less likely to publicly complain about any initial quality issues and highly motivated to publicly praise the EV virtues.
Right now investors in any company seem to be looking for a reason to either not buy or want to sell what they own. So, yes IMO that could partly be the reason for some investors; they're a skittish bunch right now.