Yes, that is Randall Park subbing in for Krasinski, much to Ryan’s confusion. The whole bit riffs on this classic cold open from the US version of The Office in which Krasinski’s Jim and Pam (Jenna Fischer) bring in their actor friend Steve (Park) to prank Dwight (Rainn Wilson).
The movie’s credits don’t mention Park (unless he has a voice role that has yet to be added to the list), but IF does, however, feature Cailey Fleming, Fiona Shaw, and the voices of Phoebe Waller-Bridge, Louis Gossett Jr. and Steve Carell alongside many more.
The story follows a girl who discovers that she can see everyone’s imaginary friends — and what she does with that superpower — as she embarks on a magical adventure to reconnect forgotten IFs with their kids.