While not all instances would directly reach each others, there is a network effect between all of them.
That said, Matrix is not a protocol interoperable with any of the above, so not part of the fediverse. It could join the fediverse, however, if it implements ActivityPub for example, or at least a subset of it. For example, just publishing the username, picture and some kind of description or start chat button.
While ActivityPub is a big player, the fedivierse is not limited to this particulat protocol. But rather to the interoperable network of protocols.
Fediverse includes: diaspora, Zot, ActivityPub, OStatus.
While not all instances would directly reach each others, there is a network effect between all of them.
That said, Matrix is not a protocol interoperable with any of the above, so not part of the fediverse. It could join the fediverse, however, if it implements ActivityPub for example, or at least a subset of it. For example, just publishing the username, picture and some kind of description or start chat button.
Source: https://fediverse.party/en/fediverse/