My guess on this is that Ashcroft had spoken to a couple of people who said it had happened and would be prepared to swear so in court. It's also the case that CMD was definitely a member of the Bullingdon Club, an organisation that does have a reputation for some unsavoury deeds - so the pig...
Truth is an absolute defence to libel: if it's true - and provably true - then the plaintiff is going to lose; malice doesn't come into it.
What you're referring to is a famous case in the US (Noonan v Staples) where an ancient law (pertaining only to Massachusetts) said that a true statement...