I don't think they meant that there should be One True Open-Source License but rather that open-source projects should pick one of the 'standard' ones (GPL, MIT, etc.) instead of writing their own.
That seems like a waste if there would be otherwise be projects around with a nonstandard license that was acceptable to a subset of users.
For example the CPython interpreter (the official interpreter for the Python language) does not use a standard license. Would you not want Python to exist?