They DO have a linux native client though, and the whole idea of using systemd according to the PR author IS because application developers can then request this information from user profile
https://github.com/systemd/systemd/pull/40954
The xdg-desktop-portal project is adding an age verification portal
(flatpak/xdg-desktop-portal#1922) that needs a data source for the user's age.
userdb already stores personal metadata (emailAddress, realName, location)
so birthDate is a natural fit.