The confusion might come from the editor appearing to accept the tagging of users. This same confusion happens when a user has no permission to tag but tags anyway.
The permission you refer to is "Maximum tag alerts per message". It does not prevent the use of typing @ user name and it suggesting users. Nor does it prevent that @ tag being converted to into [user] tags. It is there to control the number of alerts that can be generated as a result of a tag.