Well, one thought is that this blog post is from January 2019, and that a lot has changed since then. Among them is that a lot of different ActivityPub implementations have become a lot more mainstream than Diaspora.
The author's stance might of course not have changed, but it seems to me this post is a bit too outdated to enter into a current discussion. Interoperability between ActivityPub services is working fine, at least in my opinion.