Why not? it's text; you could make the same exact post on reddit and twitter (barring the length, which could be truncated in a federated context). Link aggregation is basically just microblogging with the extra constraint that the initial post has to have a link.
> Smithereen is a social network
What does this mean? Every other platform you mentioned are social networks. On all of these platforms, a user posts text or media posts. Displaying them in a different UI doesn't make them radically different.