I didn't get the email you are all referring to, but I was accepted back in December. I had my supervisor update them because of this passage in one of their interview follow-up emails.
"If you wish to remain a candidate here at Boston University, we encourage you to keep our Committee on Admissions up to date on your activities and on any substantive changes since you submitted your initial application. In particular, if you are still in school, we would appreciate seeing a copy (unofficial is fine) of your academic record through the fall and if you are involved in employment, research, or other significant work that is new, we would like to receive a letter from your research mentor or supervisor. (REMINDER: All letters of recommendation must come through AMCAS.) It would be most helpful if we can have this information prior to mid November but, if that is not feasible, send it as soon as possible. We accept update letters and unofficial transcripts via the Upload and View Documents section of myApplication."
I also sent my own update letter highlighting a job promotion, paper submission, and additional volunteer work. Could be that the dean meant personal letters don't carry much weight. Not sure how much weight supervisor letters carry, but if anything it is showing continued interest. Unfortunate that they seem to be sending mixed messages.