Also hashtags. Though I think some of this stuff came from other social networks.

Rather I think Twitter-style hashtags take inspiration from IRC channels in the format of #topic

Because channel names are not hashtags. The syntax is purely because IRC is a text-based protocol, so you need a special way to distinguish channel names from regular text.

That use— to define IRC channels— seems distinctly different than Twitter hashtags to tag individual posts. I wouldn’t be surprised if hash tags started as a nod to that, perhaps even jokingly, but I don’t think you could consider them a descendent.