In https://law.stackexchange.com/help/privileges/comment it is said that:
You can only @reply to users who have already left a comment, or edited the post. The first three non-whitespace characters in @username are used for name matching, starting from the most recent comment or edit. See How do comment replies work?
How do comment @replies work? says that:
You can use @name syntax anywhere in your comment to reply to a specific user. This will notify that user in their global inbox. There can also be notification through email if you set it up in the preferences found in your profile page. This feature is specific to comments and doesn't work in posts.
Note that the author of the post will always be notified of any new comment. You may still use it for clarity, if needed; however, if there are no comments, or only you or the author have commented on the post so far, the @name will be automatically removed from the beginning of the comment, as it adds no value. ...
Any user who has a visible (non-deleted) comment on the post.
This is usually through the @name feature. Additionally, if a user comments on their own post and there is only one other person who has previously commented on that post, then that person is also notified, even if @name is not used.
Keep in mind that the question and answers are all considered independently. For example, if Alice was the author of the question, then you cannot notify her by commenting on Bob's answer (unless Alice also participated in that answer). Similarly, you cannot notify Bob by commenting on Alice's question (unless Bob participated in the question). ...
If you are using the mobile apps, there is no autocomplete feature when typing; however, you can still autocomplete users who commented by tapping on an existing comment and using the reply button at the top.
In short the notification feature is limited and cannot be used to notify users who have not already participated in the question or answer being commented on. The autocomplete feature is still more limited, and will not be available for some people (such as those who have edited a post but not commented on it) who can be notified with the @name syntax. only one person can be notifies with this syntax in any given comment, and it only works in comments.
The comment syntax is in no way limited to Law.SE, it is the same all over SE. Questions about it can be posted on the general meta site.
I don't know exactly why this feature has the particular limitations that it does, but that is how it works.