Everyone in YC sells to each other. It's quite encouraged to buy from other YC startups and test out their tech as alpha customers because it increases the valuation of the YC network (including your own company). It's a very interesting model because as a SaaS getting the first enterprises to sign up is extremely hard, yet those add enormous equity value and provide the proof and case studies for growth at scale.

YC can be thought of as a tech startup affinity network. The"shocked-pikachu face" attitude of the author, from being surprised a napkin worked for funding to being amazed that YC-affiliated startups gave him revenue before a product is live, is a testament to the YC model. Either the author is unaware of this obvious inside-network effect or they are pretending it doesn't exist. In the real world no random enterprise cuts you a check for untested, pre-live, alpha software. It is the SaaS cross-selling YC scheme that enables this.

I'm not even saying this is necessarily a bad thing. It's just a very unique model that is a warped inversion of real-world economics, and it decreases the risk for investors provided the whole network can continue selling into each other.