The goalie was skating to the bench since the referees had made a delayed call on Philadelphia. On his way to the bench, the puck came into his vicinity, and he moved it up ice at his own blue line.
That’s cutting it close, as it’s a penalty if a goalie plays the puck outside of the defensive zone, but Miller seemed to be aware of this and waited for the puck to come to him. The charging Flyers player seemed perplexed to look up and see the goalie that far out to play the puck, and St. Louis wasn’t able to capitalize on Miller’s quick thinking.
The goalie was outstanding in the early portions of the game for the Blues, so it wasn’t surprising to see him engaged to the point of coming out to make a play on the puck before heading to the bench for the extra attacker.
(h/t to @MyRegularFace for the GIF)