this is the Mail on Sunday, not the Daily Mail - sister papers but separate operations and separate editors. The MoS editor used to edit The Tatler and has always seemed to me a bit more couth than his opposite number on the daily (or maybe I'm just a mug). He's responded appropriately, apologising promptly and suspending the two journos.
He might of course have known in advance that the gatecrashing was going to happen, but most likely not; editors don't handle the day-to-day activities of their journalists.
But as jake says, this undermines the corporate case for self-regulation.