What are the "later versions" this note is talking about? As far as I can tell, this isn't true for current/recent versions of MediaWiki, and the note it replaced in this edit (pinging @MarkAHershberger: since he's the one who made the edit) stated explicitly that $wgDebugLogFile is not required to use $wgDebugLogGroups.
Topic on Manual talk:$wgDebugLogGroups/Flow
Well, it has been two years since I made that edit and I don't remember making it or why. I've played around with the debugging logs, but not enough to know enough to say something this definitive.
I imagine I had a good reason for saying it, but I should have supported it better. Feel free to revert.