Do not read into it - a big part of an AO’s job is marketing/sales. Many schools send out holiday cards. A couple of the girls schools also sent valentines - one with chocolate! Some schools in the pack of acceptance letters to our house had hand written notes or personalized messages on the official acceptance letters - very nice gesture and reflected culture of the schools. A couple notes on the letters from HOS’s, as well as AO.
More importantly, do have your kiddo follow up with a note to the AO after your interview, virtual interview, etc.