First degree AV block exists >200msec
It is considered "marked" >300msec
The FAA didn't just pull the number out of their butts, looks like they just set the threshold to the medically defined level where it's considered significant:
https://emedicine.medscape.com/article/161829-overview
Kind of like blood pressure, normal is 120/80, but they don't yank your ticket for 130/90... you need to be significantly above normal. Diabetes is >125 blood sugar, but the AME urine test only flags something around 200.
The FAA has liberalized many medical standards in recent years (ex SSRI's), this may have just been another of those.