Something he misses is that good documentation isn't just an English translation of what the source code does, it's also a contract that describes what the source code may or may not do now and in the future.

If "the source code is the ultimate truth", then your source code is indelible---you can never change your implementation because you've given users freedom to depend upon the behaviour produced by any line of it. If you don't want people depending on implementation details, then you need documentation to hide those away.