The implication is that you can't summon servants because the "miracle that is the ritual of summoning a heroic spirit from the throne" wouldn't work. We can assume from what we know of the throne that HS exist all the same in tsuki worlds, most likely, we just can't access it (and although some fate works make it out to be plug-and-play to get your own servant, it's still supposed to be a miracle to make it happen). Ergo, no matter what they are, or who they are, they cannot be summoned (this is of course also ignoring the fact that a lot of those examples you mentioned would probably stem from some huge exception in whatever story they appear in, and not necessarily be from the throne in the first place).