The truth is not enough in defending a defamation case. You must be able to prove it is true to a third party, in this case the court. That is not always easy and it certainly is not cheap.
I work in the media industry now and I know that if someone threatens litigation for libel or defamation if they have any sort of case it will almost certainly result in a negotiated settlement out of court because the legal costs are massive.
A recent defamation case in Aus which never got to court involved the claimants lawyers sueing the radio announcer, the radio station, and the person that trained the announcer.
The other interesting issue with defamation is that defamation insurance is available but comes with some nasty provisions. I suspect that ARRSE being wise and worldly would have defamation insurance to protect themselves however the warrants would almost certainly require all the requisite warnings as well as reasonable efforts made to moderate posts. Should a claim be made against a media organisation, in this case ARRSE and they lose the case the Insurers would look to recover their costs by raising an action against the actual poster.
By the way just because you may have insurance doesn't mean you are protected.