The comments are in Japanese but if you know PHP you should be able to understand it without the comments anyway. It's a single file of less than 1K lines and written in a rather straightforward style. It's also extremely easy to setup - just edit the config parameters at the top and drop it on a webserver, and it's ready to use. Not even a database is needed.

I don't think it's "bad"; it's damn simple and works well for what it is, and contains no unnecessary complexity. No dependencies on some other huge framework, multilayered overengineering, or excessive generality. The same can't be said of the many other clones of it that were attempting to be "better designed" rewrites.

As for "maintainability" or all that other software engineering stuff: this board script doesn't really need to be maintained, because it works, and if anything needs to be changed, it's so simple that the changes can be made easily. Along the same ideas as http://suckless.org/philosophy

Also, moot was a teenager at the time.

It's kind of surprising though. I wonder what the results of a 4chan re-write by 4chan users would look like. Or even splitting out some functions for re-writing.