It's the "Public Contributions" graph that is being talked about.

The title is slightly misleading.

Nah, it's "pixel art" in the contributions graph ("Public Contributions" below the repo lists)

No, it's about the "Public Contributions" section where there are a bunch of green squares showing contributions over time. In this case, they spell out the word "hello", and there's part of a mushroom from Mario at the start as well.