The Brazilian Central Bank has authority on money transfers, so whatever regulation they want to arbitrarily impose regarding this matter, they can do.
Facebook is a registered company in Brazil, they were going to operate the payments platform through a card processor, etc, so directly or indirectly they are under the authority of the Central Bank.
EDIT: by the way, regular banks and fintechs already have to be a participant in the Central Bank's payments system in order to settle funds. PIX is (also) a 24x7 implementation of the existing electronic money transfer system which only is available on working days.
The Brazilian Central Bank has authority on money transfers, so whatever regulation they want to arbitrarily impose regarding this matter, they can do.
Facebook is a registered company in Brazil, they were going to operate the payments platform through a card processor, etc, so directly or indirectly they are under the authority of the Central Bank.
EDIT: by the way, regular banks and fintechs already have to be a participant in the Central Bank's payments system in order to settle funds. PIX is (also) a 24x7 implementation of the existing electronic money transfer system which only is available on working days.