These mobile network tracking datasets have been commercially available long beforehand. The public health agency simply put them to use to get accurate measurements of how much lockdown restrictions were actually translating into reduced movement.
Public health agencies have long been entrusted with handling personal information that is much more sensitive than movement data, so if these privacy advocates want to be taken seriously, they should do some genuine research on how this data gets used instead of simply suggesting that the public health agency must be distrusted by default.