That's a bit of an exaggeration for sure. But dust can be an issue in connectors, making the connection not as stable as expected or even break the connection o

That's a bit of an exaggeration for sure. But dust can be an issue in connectors, making the connection not as stable as expected or even break the connection on a pin if there is enough. There is also the risk of short circuit on non-coated PCB, increased ESD risk. And beyond PCB, there are a lot of other risks mitigated by clean room environments: dust on sensors, dust on bearings or gears, dust on optical cameras... And any deposit of dust will float around everywhere once in orbit, especially after being shaken and vented during launch. Could you make a satellite in your garage without a clean room environment? Yes sure, and you would even have a decent chance to have a working satellite once in orbit if you know what you are doing. Would you accept a 50% failure rate if you are flying a payload worth a few millions and need to spend a similar amount on launch cost? Probably not.