A community wiki post can still be voted on; it's just that the author doesn't gain (or lose) any reputation from it.
In the particular case you mentioned, the author made the post community wiki themselves. This is an option you can enable when writing a new post (or editing an old one of yours). Why they did this, they can only answer themselves. I've seen cases across the network where users did so entirely by accident (even though you do get a popup for confirmation).
Only ♦ moderators can remove the Community Wiki status from a post.