Maybe they don't want to confuse beginner. Saying it's C++ means some beginners will assume you can use standard lib like cout or printf.

You can use that though. Using the file system will cause runtime errors. The Arduino IDE is just a glorified build system around gcc with a package manager. I recently ported the behaviour of the IDE to Makefiles and it wasn't actually that hard.

I get why Arduino (the company) would call Arduino a language. It's supposed to be a gateway to programming, and it was for me, and I'm eternally grateful for it.

This list calling Arduino a programming language zaps a bunch of credibility from it. It's C++.