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I don't know, I didn't see the interview. But I'm assuming it doesn't look good to answer a big question like that with "I don't know", as it looks weak. I guess trying not to look weak, and come across as decisive and in control at all times, is difficult and results in a lot of bullshit. Corbyn does the polar opposite, and the result is a shambles of leadership. Maybe there's a middle ground.
He clearly didn't know at that point what he wanted to do, or what he thought was the right decision for the party. It would have been much worse to endorse one then change his mind. It must have been a very hard decision for him, as endorsing either candidate will burn alot of bridges for him in the party and lose him support among some sections of Londoners. Kahn is the first politician I've not felt embarrassed by in a long time.