Yes, it is a sub that is hosted on Reddit, but that doesn't mean anything re audience. The only way anyone would find it among all those thousands of subs on Reddit is by searching for "Mumsnet" and then they would find Mumsnet itself before they find that sub anyway.
Thanks for pointing this out, looks like there might have been a bit of misunderstanding here, we used the term 'wider' to describe an audience outside of Mumsnet, not necessarily larger. Sorry for any confusion - in short, we're just about doing what we can to make parent's lives easier.
I think whether the Reddit sub reaches a 'wider' or a 'larger' audience and compromises posters' privacy is beside the point really. It's easy for MNHQ to use that line as an easily digestible way of explaining why they are uncomfortable with the Reddit sub but it's just skirting around the real issue which is that the Reddit sub has found a genius way to completely and utterly subvert the MN Talk guidelines.
The posters on Reddit can, and frequently do,
Troll hunt (some of them are shit hot at it, frankly)
Have massive slanging matches where vicious personal attacks are thrown around with impunity
Make super sleuthy connections between user names both on MN and the Reddit sub and accuse poster X or also being Y and Z. Sometimes they are probably right, but often they are completely wrong. However if enough people shout it loudly and often enough it becomes generally accepted to be true, which is a PITA.
Drag up past posting histories and pick apart posters' stories
Doxx posters
Misrepresent posters, blatantly lie about them if they want to and in the case of posters who have had their histories removed on MN it's pretty difficult to either prove or disprove anything either way, so again something just becomes accepted as 'truth' because it says so on Reddit.
And .....and nothing. They just can. 
This is has to be a turning point for MNHQ. They are going to have to make a change somewhere if they are going to be able to take back any control of this.
Either:
MNHQ amend their Talk Guidelines so that it is no longer against the rules to openly troll hunt or to question an OP about conflicting info in past threads. It won't be pretty and it won't be 'supportive' or 'all about making parents' lives easier' but plenty of other fora manage just fine like that, Reddit being one of them.
Or:
MNHQ change their policy to say they will consistently delete ALL threads that are reported to them as having been linked to, discussed or lifted either wholly or in part by ALL third parties on ANY other internet or media platform.
Or:
MNHQ start deleting threads completely arbitrarily, without any justification or explanation to us lot. If they don't like the whiff of something going down on Reddit then ZAP! No lip service attempt at showing they've followed self-imposed strict guidelines or consistent policies on deletions. Cue some wailing accusations of a fascist dictatorship with Justine as the new Robert Mugabe, but so what? It's her site. Don't like it? Leave.
Or:
They alter this forum to allow posters to edit their posts or to delete threads they've started altogether.