Microsoft’s original source code

The screenshot of the source code at the end of the article is a ton of printed code.

How was it then entered into the Altair? Did someone have to retype it? Or was there media that predated floppies that was used?

Paul Allen entered it in front of the customer for the first run

https://paulallen.com/Futurist/Microsoft.aspx

I expect it was distributed on tape as well.

"he’d forgotten to write the bootstrap loader" He didn't load the whole program from the switches on the face, just the bootstrap that would let them feed the paper tape through the teletype/paper tape reader that was common at the time. It would take a very very long time to load the whole program by hand. See this video of a demo. https://www.youtube.com/watch?v=TxU_3dEJ2nM