Oh f*** he's use to 5 on 5 play and having 2 players not covered.
They play man-to-man AND positional, so it just gets all f***ed up at times.
The other goalies are young, or suck, or both. Allen has played for a while and knows the majority of shooter's tendencies, so that's likely what makes him confident.
He is a tremendous backup and can play more games than your typical backup can and be effective.
If the Devils get Markstrom, he doesn't need to play 60+, he could play 50 (or 42 and let Allen play 40) and be well rested for the playoffs.