I get what you're trying to do, but DM'ing implies "secret" content; since anyone can "DM" a hashtag, who controls the audience? If you DM "#NBAThreads" but we don't following each other and I DM "#NBAThreads", who sees our posts? The hashtag is metadata for a post, not an audience.
But imagine I post to the hashtag and only people who look at the hashtag see my post, what would you call that?
That's a feature of aggregation. As the poster, you shouldn't have to worry about that if you're posting publicly. It's not possible for you to predict everyone who might want to see your hashtagged post. You should add relevant tags and let the system control who the content gets routed to.