The point is (i think) the platform server is this coherent database of 'all the things' (which is a lie, because its not all the things on the internet etc.etc.). This means that there is a known state (i.e. 'global'), and subsequently a perspective on that state (the feed). Choosing your algo is therefor a differing perspective on the same thing. In Nostr, gathering all the events is just as much a part of the process as the filtering is