Looks like GHA was announced[1] around the same time as the acquisition by MS. The first commit in that repo is a year later when they opensourced it, so we can't see how it evolved before then.
For anyone who was at Github at the time, was it always written in C# or rewritten/replaced after acquisition? If "GitHub Actions is based on Visual Studio Team Foundation Server's CI" is the case, then it sounds like the latter.