I do agree that it seems to be for the connections, and for all we know they gave up 5% for that $3M.

If you do debt financing, does that look unattractive to future VC rounds?

Have any successful startups done primarily debt financing?

There is always factoring (ie. sell the receivable for a large fraction — typically 90-95% — of the face value). It eats into your profit margin, but you get the money up front, the customer pays on net30 (or whatever) terms, and you don't have to deal with billing. It works best when you need the money to buy materials or build capacity in order to fulfill the order. The longer the lead time from spending money on materials to delivery and billing, the more attractive factoring becomes.

Factoring is common and essential to certain industries like clothing that have very long lead times & where the buyers have leverage to insist on net60 or net90 terms.