Super interesting, I didn't know it was a reaction. Looking at Usenet thread screenshots, I think the UX issue with their threads is that they actually listed the title, not the actual message in a branch format. Whereas Reddit has the actual message content in a branch format.

https://en.wikipedia.org/wiki/Conversation_threading#/media/...

VS

https://preview.redd.it/r7k2gf1kav6z.png?auto=webp&s=9fd7a0b...

I think the flat sequential thread works fine with a limited number of messages. Eventually with enough replies it degrades to being very unwieldy like traditional forums.