That is the same as “How does someone without a blog make any statement of more than 140chars?”
The 140 character limit is a self-imposed one if you use only Twitter. People get around it all the time by doing multiple-tweets, or using G+ and Facebook.
Secondly, I am not against a comment system ( well actually I am, see: https://kartikprabhu.com/article/no-comment ) but, you can have both a comment form and webmention support. If that seems like too much work, then it is a judgement call. If you and your commenters are fine with putting their content on your site (along with all the log in and authentication stuff) then so be it.
It all boils down to “do you want to own your content on your site and same for your commenters?” The indiewebcamp.com answer to that is “Yes.” but again it is up to you how to do it for your site.
Kartik Prabhu
