“Well, scientists now know a lot of phone numbers,” the responsible parties might have thought, “So what?” (...) a surprising amount can be read from the data, and for some users, it can be life-threatening. (...)
And then there are several classes of data that can be uncomfortable to life-threatening for users

From further down in the article:

Approximately 30 percent of users have entered something in the “Info” field of their profile, and some reveal a lot: political views, sexual or religious orientation, confessions of drug abuse are found there, as are drug dealers who advertise their product range in this very field.

The easy accessibility of the photos would therefore have allowed the compilation of a database that, through facial recognition, often leads to the phone number and vice versa.

That isn't dystopic at all

Meta classifies the researchers' approach as "scraping" (...) We had already been working on industry-leading anti-scraping systems

Oh, the irony

Wow

That’s horrible but also wtf is up with that websites toxic popup. I thought you are supposed to be able to reject all within 2 click.

I do agree that, to a certain point, this shouldn't be possible in this volume, but also, when you add a phone number as a contact, it would show up on WhatsApp as long as the profile has public data. Anyone that saves every possible phone number would have the same result (although this could take ages, thus the issue with the scraping)