They mean that what they advocate is “free software”, not “open source”; and that what they oppose is therefore “non-free software”, not “closed source”.

Honestly, how could that be hard to understand from the context of the discussion here?

It's possible that "Open Source" here is the OSI definition, which FSF has some objections to. If "Closed Source" just means "not Open Source as OSI defines it", then it is true that some non-OSI software is "free" software. Free software still requires source to be available and that you're allowed to modify and distribute it.

The GNU philosophy believes Free Software[1] and Open Source are two different things[2].

1. https://www.gnu.org/philosophy/free-sw.en.html

2. https://www.gnu.org/philosophy/open-source-misses-the-point....