From Mattflaschen:

I'm in the process of adding link support to topic titles in Flow, which means changing the associated content formats. This isn't in Flow yet, but this is so we don't forget Pywikibot needs updating.

The mentioned change to Flow has since been merged. As such, this task is now about adding support for the new content formats in Pywikibot. In particular, content requests for formats like 'wikitext' and 'html' should map to the equivalent new formats when called on a topic title.