You need a phone number to sign up. Phone numbers are metadata that uniquely identifies people, and this data constitutes a network of connections. If this metadata is shared with the government, then it can be trivially correlated with all the other information collected about people.
It’s the volumes of phone numbers collected collectively that constitute vast amounts of metadata. Meanwhile, I’ve seen plenty of people advocate using Signal as the best option for privacy. And any time there is a criticism of Signal then then brigades of people inexplicably appear to vigorously defend it.