That is documented. Since the mercurial web interface isn't very nice to use I picked a random version. sudo 1.8.6 from 2012 writes in the man page "The sudoers file must not be world-writable,".

https://www.sudo.ws/repos/sudo/file/SUDO_1_8_6/doc/sudoers.m...

This is also a very common behaviour for security sensitive applications to check config file permissions. Another example I remember are ssh private keys.

I might be to harsh but it is not so trustworthy they still made this error and still miss the documentation.