If you had to convince folks that the fediverse / activitypub isn’t decentralization and all the perils of the former, what stories/anecdotes/tech arguments work best?

It *is* decentralization, just not the kind they imagine. In the standard web2 model you have one large organization in charge, that's more or less accountable due to it's scale, that has more or less clear rules and processes, that has publicly known people in charge.

Made up of atoms, bits, memes, and dreams. ikonoklast.twitter

In the activitypub model you have many different chieftains/landowners, not really accountable because their scale is irrelevant to the legal system, with opaque rules and processes, with opaque decision making chains of command. Who do you prefer to be in charge of your data? Anon123 or EvilCorpInc?

Ultimately the activitypub model is digital feudalism. Shitty decentralized little kingdoms where you probably have less rights than in a more modern republic.