There are numerous factors to consider in decentralisation and hosting providers is certainly one of them, but I also honestly think that it's also not as important. We can also argue that, for example, X% of all fediverse servers are hosted in the same country, or that Y% of them run the same software (and thus the protocol is effectively defined by its most dominant implementation) and so on.

The reason why I'm pushing back on this "not-quite-decentralised" narrative is because when we have truly centralised services such as Reddit and Twitter/X out there, it feels like disservice to Fediverse to call ourselves not quite decentralised when it's the most decentralised solution out there that has a significant number of active users.