Shouldn't there be a re-tag option, for community questions?
Probably not.
Community wiki questions are editable by a much larger audience than regular posts, so you have full editing privileges there.
Just as 2k+ users who get full editing privileges on all posts, you lose access to the retag option on questions you can edit completely, as it would be redundant: The edit option replaces the retag option. That's also the reason for the confused comments on your question: We 2k+ users don't have the retag link on any question.
If you only edit the tags using the edit option, it doesn't count as a full edit (e.g. towards the Editor badges).