Like WMD and Iraq. Doesn't matter if it's a lie, let's all lie together.
Credit where credit is due - the strong character on display was Corbyn, who, despite being set up to take the same position as May; with that obvious smear that took place a few weeks ago of him being a Russian agent; refused to do so. May showed her utter weakness against pressure from MI5 and the media and demonstrated she's no leader but their puppet on a string.
He was also very balanced, and did not say Russia didn't do it or anything like that, only that there is no real proof of it.