TL;DR: To create a new post anywhere in the Fediverse, you have to have an account on the host where you want to create it. (As far as I know anyway.)
To participate in comments is slightly different, and in many cases, a comment made on a separate instance (or even platform) will show up on the original instance, provided the admins of each have set up respective federation.
Nonetheless, this can also fail. Consider several people on, say, kbin.social all subscribed to the same Lemmy community on some instance or another.
They'll all see each other's comments as well as the Lemmy users' comments, and be able to interact, but if that Lemmy's admin team decides not to allow external comments to be visible, the kbin folks will be talking to each other and no-one at the Lemmy, even if their comments are in response to, and show up underneath, a Lemmy user's message.
That Lemmy user would be totally clueless unless they knew to access kbin.social and check. And no-one's going to want, or even be able, to do the rounds of all potential Federation sites to see if they have unfederated responses.
What would be nice is if it was possible to log in to one Federation site with credentials for another, or have some non-specific login details that are shared across multiple, but I suspect that's a logistical nightmare waiting to happen.
Since I'm over a thousand characters in at this point, I might as well explain that you're getting this response two weeks late(r) because your post showed up on the kbin.social sidebar for me today for some reason.
@palordrolap Oh hey, thanks for the detailed response! I actually did some exploration and learning since I made the OP, so I can bring some info to this discussion as well in case the post floats by more people!
TL;DR: To create a new post anywhere in the Fediverse, you have to have an account on the host where you want to create it.
Thankfully, this isn't the case. I think Kbin has (or had? haven't tried lately) some UI issues that prevented you from being able to properly find federated communities to submit a post to them. I made an alt account on fedia.io (linked in my profile) and I have been able to post threads on communities in other instances from there.
As far as I know, you are correct about the potential for one-way defederation causing visibility issues in the way you describe. Last I checked that was not really an intended use case for the functionality, i.e. it should normally be a mutual process, but I think that may be an area where the platforms are still experiencing growing pains.
What would be nice is if it was possible to log in to one Federation site with credentials for another, or have some non-specific login details that are shared across multiple, but I suspect that's a logistical nightmare waiting to happen.
Agreed, both that it would be nice and that it will probably never happen. :P I don't claim to know any more technical details than those broad strokes.