They could have used any Python version with one of the many version managers around or docker. I think nobody cares about what comes with the operating system nowadays.
The problem was that after 8 years there were still around libraries and frameworks that worked only with Python 2. That's a huge failure. If developers want to keep using the old stuff it means that the new one is either badly designed or badly managed.
Compare it with Ruby. There were big changes from 1.8 to 1.9 (unicode stuff among the others) and again with the 2.x series. The language mostly maintained backward compatibility and we can still write Ruby on 2.5 with the old 1.8 syntax. Community ported libraries and frameworks, started using the new features and all went well.