Ironically, if Lemmy supported decentralization of communities in this way as PieFed now does, and as I guess Reddit did iirc, then you could post to a smaller community and people who were subscribed to such a multi-community feed would be able to see it.
However, there are many features of Lemmy that are even (far) more authoritian in nature than Reddit. Lacking both a modmail and hiding the account name of the mod who removed something of yours, plus not sending you any notification about the event, are three such examples, and there are many more where that came from.
On Lemmy, as on Reddit, a mod “owns” their community, and that’s all there is to it - there is no decentralization inherent in the system, at least at the community level. Where the decentralization comes from is the ability to pack up and move elsewhere if needed. Or course, you would be able to take none of it with you, nor be able to leave a message at the old place that you had migrated. As you see, decentralization, while nowhere close to a “myth”, is quite constrained - mainly I mean, that functionality is available to admins, more than mods. So nobody can tell you what to do with the communities on your personal machine, running the Lemmy software, which is open source.
Although PieFed allows for greater levels of decentralization in numerous ways, chiefly with the Topics and now the ability for users to create their own custom ones.
Although a caveat is that “cross-posts” - even those sharing identical URLs - between multiple communities are not collapsed in the listing of posts in a feed (yet, although as Rimu said it’s a high priority to add that).
One REALLY super nice feature of PieFed is that the sidebar text is shown underneath EVERY single post. Lemmy does not do that, and especially some apps almost look like they are doing their best to outright hide that information for some reason, putting it many clicks away!?
Imagine seeing a post on All, and knowing what the exact and entire set of rules are, prior to posting (including a reply to a post, as you said a drive-by).
To be fair, someone does have to scroll down to see it. But at least it’s right there on the same page, not some whole other page entirely and buried many clicks away besides (going back and forth to writing a message that way, checking specific acronyms in the sidebar area, can get really annoying that way! in those apps that do it that way I mean, while in a browser you basically would need to open up a new tab, one for the post and a separate one for the community).
At least this seems like it would help reduce such effects? Maybe? Alternately, these feeds are basically like meta-communities themselves, created (and maintained?) by a “moderator”, so perhaps if someone did not want their community included (which seems to run counter to how many communities would want to increase rather than decrease their discoverability), they could write to the “mod” to ask that it be removed?
Alternately, perhaps communities themselves should have a “private” setting. Lemmy already has a “local-only” setting along those lines. I remember that Reddit has a bunch of opt-in features regarding discoverability, but all of this in both Lemmy and PieFed is extremely primitive in comparison. At least PieFed is moving quickly with adding new features, so for it even if not for Lemmy, there is a strong hope to see all of this that we are talking about!:-)