I've looked at the incident many times and the only thing he could have done apart from stay in goal and let Digne deal with Van Dyke, was to come out and get to the ball first.
It would've been silly to dive in and try to get his hands to the ball because VD would've clattered into him, gone over, and probably got a penalty.
He could've stayed on his feet and tried to get to the ball first, but goalies don't do that.
The funny thing is, had VD got to the ball and cut it back, Mane and Firmino were offside when the ball was lobbed into the penalty area.
Whatever, Pickford had a split second to make a decision and he showed a lot of bravery to go in as he did. Van Dyke is a big man and he was charging
in at speed.