If you see this in /etc/sudoers it is NOT a comment but an include.
For example in Linux mint sudoers.d contains the following files:
casper mintupdate README
casper for example contains the following sudoers line:
mint ALL=(ALL) NOPASSWD: ALL
So if you are messing around with sudoers or creating a new file understand that #includedir is absolutely not a comment and may be critical to properly functioning sudo functionality for your machine or user.
Of course this is contrary to most Unix based config files where often a pound # is a comment.