When we imagine the 747 in our minds, I suspect that most of us picture it fully loaded with the engines, electronics, hydraulics, seats, etc since that is what we have experience being inside and "feeling" the weight as it accelerates and decelerates.
I'm not sure what fraction of the total weight the aluminum accounts for, but the phrasing I believe is intentionally misleading.
But since the topic of the post was aluminium usage, I think the 3400 planes number is exactly spot-on. Nobody cares how much a complete Boeing weights, because that's not the topic.
