All I remember of Fred Allen is his phony Chinese accent when playing a detective.
A classic instance of offering a slap in the face with nothing to back it up. Fred Allen may not be to everyone's taste, I enjoy him but I get why others may not, but if that's all Shepherd could remember of Allen it's plain that a) Shepherd was not offering an opinion with even minimal reasonable qualification, and b) he didn't really get that Fred Allen's cumulative style wasn't anything like Paul Rhymer's cumulative style,. and that it would have been foolish to link them in the first place whatever Shepherd thought of Allen.
Why in the world do you think he had to justify that memory? Memories can be faulty, they can be incomplete, but they are not the same thing as opinions.
Shepherd wasn't hired to critique Fred Allen, and he didn't. He was hired to write about Vic & Sade.
You're making a proverbial mountain out of a molehill.