I normally have a lot of time for Owen Jones but on this I think he's got it wrong.
Firstly you can look at the political angle. For me it always rings alarm bells when people avoid defending their own position and instead start trying to throw muck at their accusers. Harman et al were in senior positions in an organisation that allowed a paedophile group to be affiliated with them for a number of years. That at least demands an explanation, if not an apology. Pointing out somebody else's wrongs does not excuse your own.
Secondly he is not comparing apples with apples. The DM stories are undoubtedly unsettling and do not sit well with them setting themselves out as some kind of moral guardians. But when you come down to it, those stories and pictures relate to post pubescent girls. The famous 'Brass-Eye' spread in the Star was shit journalism. But their headline was drawing attention to post pubescent 'attributes'. Unsavoury but the exact opposite of paedophilia. PIE on the other hand wanted the age of consent lowered to 10 years old - largely prepubescent children. That is, and was, paedophilia. The two are not the same.
I know the point you're making, and I agree that Harman et al are looking increasingly bad by refusing to admit they got it wrong. No-one has accused them of supporting paedophilia, but of being (exceptionally) negligent.
However, as cfcciom points out, above, you can't have your cake and eat it, which is something our press in general, but the Mail in particular, regular attempt with "moral" stances.
As for your distinction between pre and post-pubescent girls being the determining factor in deciding whether something or someone can be said to be paedophile, I'm far from convinced that such a distinction is as important as the fact we are referring to children. That's not to belittle your point, rather to return to Owen Jones' focus - which was I believe, to once again point up the appalling hypocrisy of that organ.