Fork length question Poseidon Redwood

Greetings. My son needs to get a new fork for his redwood. He was upgrading the brakes and crossthreaded one of the bolts. ("Dad, the front brake still sort of works" is going to win him a Darwin award, so I want to help him out).

So question for the geo nerds - stock a-c length is 420mm. They sell a carbon fork that they say WON'T fit the redwood because it is 400mm. What do you guys think? The price is right on the carbon one but if it's too big of a difference I will just buy a stock replacement for him. He's still on the 650b wheels if that helps.