Flow\Conversion\Utils::isParsoidConfigured() checks if Parsoid is configured by checking $wgVirtualRestConfig(and deprecated $wgFlowParsoidURL things).
In the consequence, a zero-configuration Parsoid is ignored partially by the editor and the editor seems to be partially broken. 'Partially' means, from HTML to wikitext converting button is enabled but the output is an error message.
I could not test strictly because I am not familiar with Flow and/or Parsoid enough. Is it right or I did something wrong?