I'm curious, it looks like the screenshot shows the author is using a Mac. But Mac OS hasn't shipped with Java in several years. So why Java? Is it simply familiarity, or are there other reasons?

Most OS's don't ship with a C compiler, go, python 3, rust, arc, etc, what does that have to do with anything?

You're conflating developer tools with a language runtime. Every popular OS ships with everything necessary to run a C, Go, or Rust binary. Python 3 and Java both require a separate runtime to be installed, but I will say that every Java CLI I've used has had terrible startup performance.