But since this is a civilian application and not military, it doesn't seem sensible to rate vulnerabilities according to military use. The intended scope of the application makes a huge difference legally and operationally and should be triaged accordingly.
It's still quite possible to discover a single or small set of existing WhatsApp users based on their phone number. So in your scenario the risk still exists, it's just more work to enumerate everyone. Everyone should still assume their phone number can be linked to their WhatsApp account.
But this has always been the case, the phone numbers are public, and phone numbers are the public key to whatsapp accounts.
Also you always could check a specific number to see if it is a whatsapp user. It is certainly an issue if a single actor can query 500 million users in a matter of minutes, and there seems to be some additional information per account like what device they are in. But these seem relatively minor.
If the application is actively distributed in a country and their usage is permitted by their Terms of Service, then yes Whatsapp is liable for the security of their users in that context. If however the application is not actively distributed in that country, and there are active measures like geolocalization (and asking the user what country they are from during signup) to avoid serving such countries, then usage in those countries is outside the scope of Whatsapp.
Furthermore Whatsapp is a civilian app and is not designed or guaranteed for military usage, it's outside the scope of whatsapp.
Can the technique be used as one tool of many (including a bullet) in order to kill someone? Yes, is this a deadly security vulnerability? No, of course not, that's reaching, I'm not sure what would compel these exaggerations, maybe the larping, maybe its a general hatred towards whatsapp and you just jump on any opportunity to release your pent up anger.
It's worth noting that there's a gap between the security capabilities of whatsapp and the security capabilities they are legally required to have. Whatsapp will no doubt patch this small issue and keep that gap, but WA as it stands is one of the most secure and widely used applications in the world, has had an almost impollute historical record which is why billions of users trust the application with personal and professional secrets.
P.S: Also, you always could find out if a phone number is a whatsapp user individually, just add them on whatsapp and try to message them.
Do you work for Meta?
People don't use WhatsApp because it's so secure. In certain countries people started using it because it was the first app that was cheaper than SMS and now they use it because everybody else is still using it. There is no other significant reason.
They have a history of security issues going back to 2011 when you could take over other peoples account. Today is just the last story of this ugly and leaking brother to Signal. The actually "most secure" app out there.
What are you talking about? Like what is even the mechanism for your concern?
This is an open endpoint / not a part of the design that is intended to be confidential. If you suspected any particular individual you could always check if their phone number had a WA account.