I'm still not following you. What clues did he provide?
You're the one who wrote about the "attention-drawing features" of the message.
Handwriting Analysis Details
(last updated June 6, 2004)
Many people have stated that they do not believe that a child wrote the letters. But their statements really have nothing to do with the handwriting. It’s just people voicing an unwillingness to accept or believe that an adult would use a child that way - or that the terrorist would risk using a child that way. These are beliefs and have nothing to do with facts.”
Ah! Yes, countless people noticed the highlighted characters. But how many of those people immediately began looking for a hidden message?
The vast majority believed that Muslim terrorists were behind the anthrax letters. Why would a Muslim terrorist put a hidden message that requires knowledge of condons in a threat message, particularly if the terrorist couldn't even spell "penacilin" correctly?
How many people have read GEB? Lots. (I have a copy.) How many people who have read GEB would make the connection between the code described on page 404 and the code in the letters? The answer appears to be 1. And, that person only made the connection AFTER Ivins was observed throwing away his copy of GEB and a magazine about condons.
Ivins gave a copy of GEB to another scientist at USAMRIID, and when the scientist didn't read it, Ivins asked for the book back.
Your 20/20 hindsight says it's so obvious that every biologist in the world should have seen it. But they didn't. Why?
They didn't see it because it didn't occur to them to look for it.
The "attention-drawing features" attracted the attention of countless people. Many figured it had something to do with Mohammed Atta, one of the 9/11 hijackers. I thought it was "doodling" of some kind, but why would a terrorist doodle on a threat letter?
You need to view things the way they were viewed BEFORE the answers were found. 20/20 hindsight assumes that everyone in the world was smart enough to figure out the facts BEFORE the facts were actually discovered by anyone.