As he presents himself in the book, at least, he was basically a dilettante, one of many who got sent to the GC&CS at Bletchley (the people who get described in Kahn 1991 or Winterbotham or Calvorcessi in their explanations of who worked at Hut 6 or 8 as 'linguists, mathematicians, people who wrote crossword puzzles' types). He flunked out of GC&CS, however, and only got the job with SOE by the skin of his teeth (the general in charge of SOE wanted him to decrypt an actual operational message but forgot to give him the key- the general wanted to see how fast he was at doing the double transposition cipher compared to a clerk, but instead Marks over the course of a day's work cracked the actually sent operational message without the key, which impressed/scared the general far more and got him the job).
But he had just rapid wartime training of learning by doing, and he was largely by himself as the only cryptographer in SOE, at least as he depicts it, so it highly plausible to me that he missed a lot of things that were widely known in the broader cryptography community. (It was because he was so isolated from the rest of the British crypto community that he ended up allowed to write public memoirs, I suspect. Wiki says he wrote it in the early 1980's and wasn't allowed to publish until 1998.)
But he had just rapid wartime training of learning by doing, and he was largely by himself as the only cryptographer in SOE, at least as he depicts it, so it highly plausible to me that he missed a lot of things that were widely known in the broader cryptography community. (It was because he was so isolated from the rest of the British crypto community that he ended up allowed to write public memoirs, I suspect. Wiki says he wrote it in the early 1980's and wasn't allowed to publish until 1998.)