Defederating from Meta as a solution is stupid - Meta can (and will if they actually care enough) just rejoin undercover.I agree with the rest of your post but I suspect that in this instance Meta, a large public company, that might leery about doing something that potentially embarassing.
But your basic point remains true. A wide variety of companies, many bad actor, are going to be scraping, processing and connecting any data anyone makes puts online and that will happen whether Meta joins the fediverse or not. If Meta wanted fediverse data bad, they'd likely just buy it.
Indeed, all the hand wringing about Meta in particular in the article and here seems deeply confused - of course Meta isn't the only problematic actor out there. Indeed, the anonymity-problem of all the walled-gardens is that they explicitly attempt to stop online anonymity in various overt ways. But everywhere entities are trying to deanonymize covertly and these can be at least as bad.
The main thing is that anyone wanting anonymity needs to take active measures to achieve it. And these measures vary on how visible you are and effectively how much your enemies are capable and interested in you. Those talking anonymity would do best educating people in this.