Having "add new post" in the header on kbin it's definitely something that will trip up people coming from Reddit. You need to add a new "article" which isn't very intuitive
Microblogs are like tweets. I think posts from people you follow on Mastodon and similar federated microblogging platforms should appear there. I wish there was the option to merge the microblog and magazine feed. I don't think having them separated is necessary on a platform like this.
It's for Mastodon compatibility. Articles are like Reddit posts and microblogs are like tweets. You can post either from Kbin. Your articles will show up as community posts on Lemmy, and your microblogs will show up as toots on Mastodon.