"true decentralisation" is a technical problem, but ultimately boils down to a huge marketing game.
both activitypub and atproto have their pros and cons. both struggle with decentralisation for a number of technological and cultural reasons.
atproto (bluesky, etc.) is decentralised because it enables any user to host their own canonical data, regardless of where it's utilised, entirely on their own hardware.
it is centralised because the vast majority of atproto usage is consolidated to bluesky, and its own centralised appview.
the cultural issue is that most people still call the entire platform "bluesky", as for most people that is their only presence on atproto. they have no idea about the underlying protocol. this is a black box for all but its most technical users.
activitypub ("fedi", mastodon, etc.) is decentralised because activitypub's federation makes it much easier to build a decentralised social network composed of personal/community instances.
it is centralised because the requirements of running an instance can be incredibly high-bar, resulting in most instances being for communities, and not single users. user data is also generally not portable; it lives and dies with the instance.
the cultural issue is that the whole platform is still called "mastodon" by most, despite them only being the flagship instance. further, a very large portion of activitypub users are housed on mastodon(dot)social, or neighbouring mastodon-branded instances.
all this to say that "my network is more decentralised than your network" is a silly, reductive argument and a waste of time. we'd be much better off learning from what each implementation has done well, and striving towards a "best of both worlds" approach.