Decided I should post a finale/follow up to my OP while info is fresh, just in case someone stumbles onto this thread in coming years. Qualifier: I'm recapping what I was told by an 18YO. Sounds like the letter was not as big a deal as we originally thought. Coach only wanted to make sure that parents knew their kid was having these recruiting discussions (I'll note letter was addressed to son and not us). Son was able to get in touch with Coach several days after he received the letter arrived. By phone. In addition to info about the letter, son asked if admissions had received his updates (sent in latest grades, which were strong and included several AP classes including the required AP Physics). Coach said yes but that he was past the point in admissions where grades mattered and that he was in the 'faculty review' round (faculty are part of the admissions process here, which is somewhat unique). Definitely not a guarantee of anything but all good news.
My biggest takeaway from the process - students should establish a voice / video call relationship with Coach early in the process. He made several suggestions about the application and essays, suggested some admissions seminars to attend, and pointed him to the regional representative to contact prior to application submission. I don't know how much this stuff mattered but it definitely let my son build a relationship with Coach, and let him learn about what to expect from Coach in the future in terms of response time, level of interest, etc. Do not expect important info to be communicated via text or other messaging. They're not going to put it in writing. Much more willing to give the student 'unoffical' info on a voice call, esp if they've developed that relationship. Good luck!