Apparently you will not be impressed because it is someone who is very distasteful to you.
The Term "Constitutional Historian" means some one who studies the Constitution.
Byrd has been a legislator since 1946 in the W VA house and has cast just shy of 20,000 votes in the US Senate.
He has a B. A. Degree in Political Science and Law Degree (J.D.), both of which he earned while being a member of U S Senate.
My guess is with a 60 year career as a legislator, two degrees and an obvious interest in and passion for the subject, it is not at all outrageous to consider him a "Constitutional Historian." Some fellow Senators call him a "walking encyclopedia" of such matters which strikes me as a very specific complement - they could have said, "he is well versed" or "he sure knows his stuff."
No, I'm not impressed with his credentials as a "constitutional historian" because he has none. Displayed none to my knowledge, anyway. That he's been in the Senate for a long time is nice (I'm curious how many votes Teddy Kennedy has to cast before he too becomes an historian), and that he has the constitution memorized is nice, but that no more makes him an expert on constitutional history than memorizing the back of a baseball card makes one a go-to guy on how to play baseball.