No, you’re not quite understanding what ActivityPub is. The data under all the fediverse services is not the same infrastructure at all. The communication between those various services just uses the same language (ActivityPub). Those various services can interpret and store (or ignore) ActivityPub messages any way they want. Service instances add another layer to the whole thing as well.
In order for an “everything app” to be successful (if you buy the argument that it feasibly can be), it would have to be a centralized service. Decentralization, by its very nature, encourages the opposite of that – want to make some niche service because existing services don’t satisfy some fringe need you have, but still want to interact with others on other platforms? You can do that with the fediverse. But that also means your new service isn’t part of an “everything app”… it just can potentially talk to one that might exist.
What a dumb article. Sounds like an old C graybeard who’s never understood the point of proper type safety or readable code. None of the performance gains the author talks about actually matter, whereas the entire point of clean code is to make it easier to read and maintain by other programmers. Let’s also not forget this important quote from Donald Knuth: “premature optimization is the root of all evil”.
Simply put, unless you’re working in extremely resource-constrained systems, or have some code snippet being run an incredibly large number of times over a humongous amount of data, these kinds of performance optimizations simply don’t matter and you get more benefit from writing the code in a way that reduces bugs and is easier to read. Heck, most of the time compiler optimizations make this entire argument moot anyway.