You can use @meId and @meUserName for quite some time now in commands. However these tokens only work in delegated mode.
When you try to use these tokens using application permissions, a value will be used from the access token and the API request will fail.

Example @meId:

Example @meUserName:

In both cases it would be much clearer if we throw a clean error saying that you can't use these tokens if you are using application permissions.