My opinion is as good as yours Glidrose (in my opinion anyway).
Um... did anybody say otherwise? The author's point is that Fleming is unique in how many considerably more accomplished novelists have followed in his footsteps.
The author's point is to imply that Fleming was a hack who had proper novelists following in his wake.
Yes, well, peacocks tend to fare poorly in poultry shows. No matter, the much better writers/novelists were handicapped from the outset in that they were trying to write a Fleming pastiche. I can only image the results if Fleming were still alive and tried writing Birdsong based on Faulk's outline, or "Any Human Heart" based on Boyd's.
Which only proves that a thriller writer would be no more proficient at writing middlebrow realistic novels (which are really just another genre) than a middlebrow realistic novelist would be at writing thrillers. Should a sushi chef be downgraded because he can't make a good pizza?
If Dustin Hoffman is such a fantastic actor then why could he never be as good a James Bond as George Lazenby?
For the same reason that he would be miscast in other adventure/action leading man roles. It's also worth pointing out the actors widely regarded as the best Bonds--Connery and Craig--happen to be pretty in other roles too.
Fleming did not write for the "A audience". He wrote for "warm-blooded, heterosexuals to read on trains." Yes, some very literary people read and enjoyed Fleming's novels. But not because it was "A level" fiction but because it was pulp with some literary flair.
The A-audience quote comes from Fleming's letter debating whether he should allow the Express to run comic strips. Undoubtedly some members of that audience had warm-blood and read thrillers on trains...