Not quite at that time, the Powerbook G4 wasn't announced until January 9, 2001, at the time it was the Powerbook G3 Pismo that was Apple's main pro notebook.
I don't think the magnesium case played much a role in the weight difference between the 12.1" VAIO N-505 (3.75 lbs / 1.7 kg) vs the Powerbook G4 (5.3 lbs / 2.4 kg), so much as the bigger 15.2" screen and integrated optical drive in the Powerbook (which at the time was remarkably unbulky for its size)
