It seems to be working now, I can see the post that you made on both views of the community:
Normally, any posts from before lemmy.ca learned about the community would not have federated over. A lot of Lemmy and Piefed instances make use of tools like lemmy-federate in order to automate that process. I'm not familiar with if Mbin has something like that.
It is possible to manually pull individual posts, by pasting the link into the search bar on lemmy.ca. For example, I was able to add the two older posts by copying their original links (ex. https://thebrainbin.org/m/Sailing/t/1849603/Would-this-photo-of-the-MIT-sailing-pavilion-be-a#comments), and pasting them into the search here: https://lemmy.ca/search