No. I remember my wife bought a Sony Vaio N-505 back in 2000. An amazing machine with a magnesium case, very thin and very lightweight. Only the 10.4" screen was pretty crappy (but they all were back then).
Apple had the Powerbook G4 at that time which was a very high-end machine but much much bulkier of course.
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)
Apple had the Powerbook G4 at that time which was a very high-end machine but much much bulkier of course.