The author of this article doesn't know what they are talking about.

First of all it confuses "decentralized" with "distributed". Generally speaking "decentralized" is a broader term that includes "federated", but specifically when talking about where data resides you can well argue that Matrix is less decentralized than Lemmy, as Matrix.org siphons up pretty much the entire network and most accounts reside on matrix.org by a long run.

Secondly, that ActivityPub mandates a single source of truth is a deliberate choice and not a bug. Principally AP could do the same as Matrix as communities are already cached on other instances. The problem is that you end up with extreme netsplit and moderation issues if you allow multiple source of truth in such a network. Matrix tries to prevent this by forcing everyone into a DAG (git like data structure), but this is a hugely inefficient and privacy problematic "solution" to what can be much easier solved by having an main original community that other instances can refer to.

I also think that is is myopic to only look at communities and not the overall network. If the origins of communities are widely distributed over a network, the network itself is decentralized even if individual communities are not.