there's a large security perimeter that's already been breached, and the video shows one chokepoint of the perimeter that hasn't yet been breached.
The officers at that checkpoint know what the situation is: They aren't going to do any good in an open space surrounded by a mob of thousands. They are moving back to a more defensible position (presumably where they can buy time to evacuate the Congress floors). They are calmly walking because breaking into a jog could incite the mob further.
I don't see them opening the gates.