I think we are talking about different things here?
The single link you see on Twitter just takes you to Reddit's website. It doesn't let you take part in Reddit from twitter.
If you just want to share a link to Kbin from Mastodon, sure that's easy, just post the link.
What I'm talking about is interaction across different platforms, replying to a Kbin thread from a Mastodon account. This is impossible on Twitter, because Twitter and Reddit don't have interoperability.