If it’s on Github doesn’t that make it OS?

No, when talking about open source software, people typically refer to a definition along the lines of the Open Source Initiative’s Open Source Definition. To distinguish this from software that you can only see the source (but don’t have rights to copy and modify it), they’ll use the term Source Available Software.

I don’t really know about the software you guys were talking about, but the repositories I looked at used the MIT license, which is OSI approved. However, that might not be all of the code they use. It’s not uncommon for a company to open source a “base” version, but they deploy a version that’s altered from that (I’ve got no clue whether they do or don’t).

Yeah, it’s like saying I can “compress” a png of the Mona Lisa to just the string “Mona Lisa” because I have a database of art.