Buck2: Our open source build system

I'm missing some historical context here. This article goes out of its way to compare and contrast with Bazel. Even the usage conventions, build syntax (Starlark), and RBE API are the same as in Bazel.

Did FB fork Bazel in the early days but retain basically everything about it except the name? Why didn't they just...adopt Bazel, and contribute to it like any other open source project?

At the time that FB started writing Buck, Bazel was not open source. I believe it did exist as Blaze internally at Google before FB started writing Buck. Facebook open sourced Buck before Google open sourced Blaze as Bazel.

Over time Facebook has been working to align Buck with Bazel, e.g. the conversion to Starlark syntax so tools such as Buildozer work on both systems. I believe Buck2 also now uses the same remote execution APIs as Bazel, but don't quote me on that.