There's like 3000 people in there. Sometimes shitloads of conversations are happening at once. The people that refuse to use threads aren't cool, they're just co-opting literally the entire channel for just their conversation.
I see it as a forum, where you browse threads and click into the ones you want to discuss in. I guess other people see it as a bloodless Colosseum, may the loudest win.
If there is already a lively topic ongoing in a room and I need to jump in with another topic, I write a message and say "Hey I have a question, can someone help me?" and then I immediately start a thread with myself so people see that it is a thread and click it to respond rather than just spamming the room.
I have always wanted to try out twistapp and see how it is with their "Thread first" UX mentality.
In my current team that uses Slack, I just made it a policy that all conversations have to be in threads. If they're not I just flip a shit until people give up and make threads again lol. Most people don't care enough to retaliate, so that happened a few times and then everybody got used to the system; this is basically an approximation of Twist now (where the destructive behavior is super easy to commit... sigh)
The rest is a cultural problem that exists will all communication channels (including email). Ask people to be mindful and not use @channel all of the time (you can disable @channel if it is abused). Ask people to keep their chats to relevant channels. e.g. Daily interesting links do not belong in #general, please keep it to #random. Stuff like that.
Also, encourage people to move long complicated topics to a thread (to not annoy everyone in the room). Or ask them to compose a larger email or wiki page on the topic.