Its not that they still had python 2, its that the binary "python" referred to python 2 on ubuntu (it might even still be like this) while other distros had it pointed to python 3.
Given that python versions are incompatible by design you should probably explicitly refer to the version your code supports. At least that is my takeaway from this mess.
This whole situation is a bad trap for novices, given how many tutorials, class slides, etc ask students to copy/paste various invocations of pip and python that may or may not work verbatim on their distro.
