It's a slight overstatement for rhetorical purposes. Supporting python3 is not a difficult problem even for large libraries with single maintainers; as I noted, automatic translation tools exist.
Yes, 2to3, tox, etc exist, but of course in practice it's not necessarily that simple, even for bigger libraries. Take Airbnb's Airflow for example.
Yes, there are some python2 features that work differently in python3. But python3 has existed for a decade now, and there aren't real benefits to using forward-incompatible methods. Writing code that won't work in both is a mistake.