Documenting quickly as I'm currently busy with other stuff (namely
#982716) and I don't want that I forget about it:

Citing from
https://www.debian.org/doc/manuals/aptitude/ch02s03s05.en.html:

There is no "non-free" section. "non-free" is a separate archive.

Nevertheless the example itself is still correct as these packages do
have "Section: non-free/$section" in their meta-data. But "the non-free
section" is still wrong.