This is a misunderstanding of the issue. ActivityPub the standard will of course survive the extension of use to Meta. ActivityPub is just a protocol. There are reasonable concerns about the distortion to the priorities and focus of the projects development long term if Meta gets more involved but those are not unresolvable, and have been seen in other open source projects when big business gets involved.
People's objections are also due to how federating with Meta's software, content and commercial priorities will distort the existing communities in Fediverse/Federation/Threadiverse.
While the technology allows the Fediverse to exist, one federation is not the be all and end all of the fediverse; it is understandable that existing parts of the Fediverse might not want to federate with Meta. There can be multiple federations within the fediverse; it doesn't have to be one joined up mass and it's not likely it's going to be one single complete federation long term. People will naturally form interconnected communities around different ethoses, priorities etc. I think there will likely be one big "primary" part of the fediverse just due to shear size and mass of content; but it doesn't follow that it needs to be linked with Meta's implemenation nor that Meta's implementation would damage a separate fediverse. That is what people are pushing against - they do now want the main threadiverse or other elements of the fediverse to be subsumed and lose it's identity due to being swamped by Meta's social media platforms.
It's about the separation between the fediverse as a technological solution and the fediverse as a philosophical solution.