Well.....I don't know why you included Twitter on that list, as they've NEVER been part of the fediverse.
Threads is fully integrated. You can personally block them from your end, but thats all you.
It would be like saying "Dominos doesn't make pizza. It has never been a pizza company". With your logic being that you don't like their pizza. Doesn't make it true just because YOU don't eat the pizza.
Bluesky I hear conflicting reports on. Some people say it is, because it can be, others say it's not, because it's not official. I get both sides on this.
But the last part......is objectively not true. It happrns to work that way FOR NOW. It just isn't profitable enough for the major players to sink any real resources into.
The fact that it's adfree has more to do with the fact that 60k people on all of Lemmy with most instances having a few hundred people "on" it, and also advertising companies not understanding the concept of federation.
I could start my own instance, and sell ads to corporate overlords. The biggest problem I'd face is the idea of trying to convince any company with money to spend that money on me putting an ad on for such a small audience.
If/when the fediverse ever gains momentum and becomes mainstream, you can guarentee that ads will be everywhere.
Because nobody owns the fediverse. Which means if I sell an ad on my instance, all federated instances will see the ad. Sure, you could defederate from my instance. But what would happen right now if lemmy.world sold ads? Is every instance going to defederate from the biggest instance, with the majority of communities? That would essentially break the fediverse.
We're all on a service that you think is immune to centralization, but forgot the core concept that humans like to socially congragate. Which means it's inevitable that there will always be one big dominant instance. Which means if this thing ever goes mainstream, the ads are coming, and they'll be on all the big instances.