Do you think most people who use iphones, Android phones, and MS Office read the documentation of this software?
You are free to do so , but most people just want to get on their lives. I bet most people do not even read their car manual.
In the article you show that 10% of people use Safari, in other words, Codebook is not the right solution for 90% of desktop users. Hence why I say, its a deal breaker for 90% of desktop users. If it best suits any one, by all means use it but I can not generally recommend it as other password managers have superior capabilities.
Your original point was "Codebook is too obscure and I wouldn't say its the most pleasant thing to use". I believe you were referring to Secret Agent and the way it supports filling in passwords in browsers other than Safari.
I don't feel it's too obscure. They do advertise their "global keyboard shortcut" on their main web page; that provides a link to the details of Secret Agent. Also, the preferences of Codebook have a checkbox to enable the keyboard shortcut, which must turn on Secret Agent. That seems like a basic thing a user would set and that would enable password filling in other browsers.
I have no comment on it not being very pleasant to use since I've never tried Codebook. I'll trust your judgement on that one. If that is true, then that would be a major impediment to me using Codebook, since I spend most of my time in Firefox. Also, I sometimes use Windows and expect password filling to work in browsers. Secret Agent is the way they support that platform. If it's yucky, fuggedaboudit.
It's a significant advantage to me that 1Password works consistently on Windows, Mac, and Linux - and in all major browsers. And, even though I find their browser extension to be problematic, it is pretty full featured and works well most of the time.
1Password also supports a pretty powerful global autofill feature with a keyboard shortcut. I don't know if Secret Agent is as powerful. 1Password's global autofill even provides the password if I need it when I run "sudo" in a terminal or am prompted by the OS for administrator credentials. Setting that up is a bit obscure.