UEFA is only a subsidiary of FIFA in terms of the administrative hierarchy of the game. FIFA is not in charge of UEFA when it comes to the laws or their application, the IFAB is.
As far as I can tell, looking at things from the outside, it seems the IFAB is not requiring that the new, stricter interpretation of of goalkeeper movement be applied to games played under the old laws. Once UEFA starts operating tournaments under the 2019/20 Laws, I'd expect to see the same standards in force when VAR is checking on penalties.