This was the nicest unofficial waitlist/sort of rejection letter I've gotten this MFA season. Just made my day! Amazing what just the smallest encouragement can do for the soul.
As you suspected you were in our Top Ten and others who had offers before you have delayed and dawdled and held out for better, the usual early April antics before the deadline. You had better take another acceptance if it pleases you; things look like it might not happen here. And no, the missing rec letter was of no consequence, not after we read your wonderful submission.
We have only six places and, out of 150 applications, know that you sailed to the top of our pile. I hope you got in lots of programs; I'm sure we aren't the only ones with eyes who can read.