But when you ask "Why are people concerned about Meta using ActivityPub?" this is why.
It's still worth asking to clarify rather than guess, I wasn't sure what are we discussing and I got 3 different answers so far anyway - it's either about ruining the ethos, stealing the data and/or changing the protocol.
Let's just hope fediverse manages to get big enough before meta launches this so it manages to grow from the influx of new users rather than being stomped over completely. Or alternatively we get other big players into the fediverse too like google or MS, at which point they can fight between each other if they want to.