Only true for Pi products. If you're willing to spend $7 you can get esp8266 nodemcu hardware delivered from amazon prime in arbitrary quantity.
Theres a problem where no matter how many times its stated, "you can buy ONE if you pay six times MSRP or live within driving range of the one retail store on the continent selling them or if you import from the UK or eastern europe" vs I need six for an IoT experiment involving micropython and I2C environmental sensors. I can't buy six $5 pi or even six $10 pi but I can have six ESP8266 boards by thursday afternoon for $35 total. Which is less than single pi zero plus "development kit of parts" costs, ironically.
A $5 computer that's impossible to actually purchase is much less useful in practice than a $7 computer delivered in two days by amazon prime in any quantity I'm willing to pay for.
Its interesting that above $40 or so there is an infinite supply of rasp pi 3 and beaglebone blacks. Everything is widely available except for cheap pi.