Zebra = black and white stripes. No green or red lights!
It seems a tad harsh to call that a SERIOUS fault, if it was exactly as described, if the pedestrians showed no likelihood or wish to cross, but, if you didn't demonstrate consideration for pedestrians at a zebra, then maybe the examiner felt there was a general too keen to make progress thing going on, and maybe that's just the one he hung his hat on?
Of course he may have been over zealous. It's amazing how much more notice of things you get when you're riding behind observing someone. A couple of seconds can make all the difference.
In some parts of the country, Stockport for one, no motorist ever seems to slow for a zebra - I don't know how any of them ever got their licences. In other parts, like Oldham, motorists are brilliant at zebras but instead they straight-line every mini roundabout.
Take it on the chin, make sure you demonstrate due consideration and anticipation wherever you can, it will put you in good stead for a lifetime, and help your examiner pass you next time.