The person who spend a lot of time inside the code base obviously finds it logical and knows his way through the codebase and where to make changes what to avoid etc. For that person, the codebase is maintainable. It is different for a person who's new to the codebase. If that person has to spend a lot of time figuring out where what is and most importantly: why, it's already a sign things aren't as maintainable as it should.
I have to say that I disagree quite strongly with this definition.
Maintainable is a value that can only be applied by someone who is familiar with the codebase. If that someone find it hard to work on the codebase, it is hard to maintain. If someone with no knowledge of a codebase find it hard to work with it, tough luck, but that doesn’t say anything about the maintainability of a code base.
A codebase is more than just a code sample, it is conventions, ways of doing things, overall architecture and a sense of how to do things.
Now, there is a separate value, related to how easy it is to bring someone up to speed on the codebase, but that does not relate directly to maintainability.