Possible reason: It is actually intentionally avoided to use the concept of "ownership", since those advocating for "data ownership" generally mean things like "if people own their data, that means they can give it to companies and then that company can do what it wants with the data, since it is its property", which is more or less the opposite of what the goal of this privacy regulation is. The term is used sometimes in more general material, but it is problematic framing. People have rights to their data and how it is used, but they don't derive from ownership.

So in that particular sense, it is even stronger than ownership, at least in terms of preventing facebook from ever becoming the owner. (Not in the sense of having all of the rights usually associated with ownership, which include selling)

The article linked doesn't back up the claim that GDPR makes the user the owner of the data companies collect. Being entitled to obtaining a copy of a company's records about the user is a very different thing than making the user the owner of that data.