Keynes' ideas have been adapted to a range of different forms, but are more about circumstances where the economy is in a deep recession and there is some restriction preventing the economy from utilising all its resources. Here debt financed spending may have demand-side benefits by stimulating yet more demand. This only works at the aggregate economy level, so Elon would have to launch a lot of satellites to benefit from it!
To be fair he's talking about tripling the number of satellites in service with his constellation alone. Within the industry spacex would become the primary supplier and the primary consumer of rocket launches.
I hadn't heard of Arrows learning by doing but that makes sense as well for this discussion.