Stephen Hackett hauled out his G4 Cube to show the similarities and differences between the design of vents on the Mac Pro and the Pro Display XDR and two G4-era models:
While the design from the outside is very similar to the Mac Pro, to my eye, it is not quite the same. The new machines uses hemispherical cuts, while the design on the Cube is created by two overlapping flat components. There are no fancy hemispherical cuts here.
Looks to me like Apple was experimenting with this design way back then, but in an era where its skill in working with metals was not remotely close to where it is today.