I still see three possible ways for Embrace, Extend, Extinguish.

The first is the Android way: while Android is FLOSS on paper, Google makes sure to cram as much important functionality as possible into their proprietary and closed Play Services blob. I have no intimate knowledge of ActivityPub, but I reckon it would be relatively easy to contribute a tailored modules wrapper under any required GPL licence, and then use that wrapper as a gateway to closed-source extensions.

The second is the Gmail way as described by Emi here: build a portal based on established standards that becomes hugely popular because of its ease of use and feature set, then start sneaking in cool non-standard features that only work inside your walled garden.

The third is the Microsoft Java way: build your closed-source clone from scratch and make it just incompatible enough for non-technical users to think that the original implementation is broken.