Someone had to read through GEB and realize that the image on page 404 was similar to the highlighted characters in the media letter. Then they'd have to realize that the codons described in the magazine article could be the code.
They didn't need GEB. All they had to do is:
1. Notice the highlighted A's and T's.
2. Remember that A's and T's are used in the description of DNA sequences.
3. Construct the codons and translate to a protein sequence. (This would be a standard thing to do for anyone with the slightest background with DNA.)
I hope some FBI agent writes a book some day about how they figured that out.
If they had already noticed the highlighted A's and T's, it doesn't speak well of the FBI that they didn't think of a DNA connection, especially when all their suspects were biologists.
That's your 20/20 hindsight at work. Things always seem "obvious" after someone figures them out.
The letters were printed in the media. Every scientist at USAMRIID, and possibly every microbiologist in the world saw copies of the letters somewhere, yet no one ever contacted the FBI to explain, "it's obviously a code using condons and here is what it means.
Your reasoning is pure 20/20 hindsight.
Besides, there's a lot more to DNA than A's and T's. Even condons also contain G's and C's. The FBI reports includes an interview with a scientist at USAMRIID after the FBI figured out what the A's and T's in the media letter were all about. FBI file #847547 contains the interview on pages 19-23. Even after it's explained what the code is, the scientist still suggests that the code could be "ATTACCA," which is Italian for "attack," even though no C's are highlighted. And he suggests reading the letters right to left, the way Arabic is written. He seems convinced that a Muslim wrote the letters, and he's looking for proof of it - ignoring what the FBI is telling him.
How many people saw the letters in the media, and how many figured out the code before the FBI? Doesn't your line of reasoning say it should be at least 50,000 or so - the number of microbiologists belonging to the American Society of Microbiologists?