Maybe it just sounds that way to engineers then; I'm an engineer, and I read it the same way GP did. "on" is the preposition I'd use if I were describing the engineers as working on Github (as I just did right now)
- Github is the platform/medium/support facilitating work - Github is the target/aim/focus of the work
E.g. 'Working on a table' means I could be making a table or I am using a table to work on something.
Having said that we know Microsoft owns Github. If the same article had come out in relation to Apple we'd immediately know what it was talking about without any confusion.
Post from 2015: https://jeffwilcox.blog/2015/11/azure-on-github/
I do realize it's probably very unlikely anyone here read that post. Happy to see if the OP wants to clarify better.