Personally, I wouldn’t make too much of the difference between a reporter describing something as a letter vs describing it as a memorandum. (It’s actually in the form of a memorandum of law, not a brief.)
I read Will’s article and thought he missed the mark on the insinuation that Barr must have had an inside scoop on the Special Counsel to know that Mueller (Weissman) would be pursuing an unusual novel expanded theory of what legally constitutes obstruction of justice.
This was publicly known before that, about the obstruction theory being pushed by the SC. In fact, I think Rudy Giuliani may have objected to that theory in a letter to the SC which became public, IIRC. I think I may even have commented on here early on after the SC was appointed that the reason Dreeben was brought on board may have been because he was known to favor that expansive theory and is considered an authority on substantive federal criminal law.