> You don’t need that many customers to design and ship an SBC.

Right. An SoC takes five years and tens of millions of dollars.

An SBC using that SoC can be designed and made in three months for the cost of a small team's salary -- possibly even just one person e.g. see Paul Stoffregen of Teensy fame, or Jonathan Oxer at Freetronics. I'll bet the teams at Sipeed, Banana Pi, Orange Pi etc are pretty small too.