It's not centralization if you can set up an account tied to your own domain name (similar to an email address for your own domain) without needing to self-host an instance.
No, in the same sense as creating a Bitcoin wallet "sounds like centralization". "True" decentralization does not give any power at all to instances, the same way a Bitcoin node has no power at all over the Blockchain and it would be a bug if they did.
Sounds like centralization. Now if Activity Pub goes rouge ALL instances are affected.
Making accounts on your favorite instance and communicating with other instances prevents this, in theory. You may need to make a new account if Lemmy dies but you should (again in theory) be able to move your content seemlessly to a new instance. Becsuse Lemmy isn't facilitating the content, simply providing a view for itz based on its usage of ActivityPub's API