With a mattermost setup with hundreds of users, you get a lot of username matches when you type @, however if you're in a channel with only a small subset of the hundred users, you still need to narrow down the list by typing in more characters than necessary to match the name you want. For improved efficiency the list of users to match against shoulld be limited to the users currently on the channel.

This limit should of course not be applied if you are doing /msg @someone, and possibly not even when you mention @someone somewhere inside your post (as opposed to at the start of the line).