The base CT6 is RWD only and 2.0 engine (with Auto box) weighs 1659kg 'curb' weight, there isn't really a comparable model in the 7-er range, but a 2.0 RWD Auto 5-er weighs 1690kg (inc. 75kg driver) i.e. 1615kg 'curb', so that would be lighter. If we subtract the 65kg difference between a 4 cylinder 5-er, and a 6 cylinder 5-er, from the weight of a 6 cylinder 7-er, then we'd get a 'curb' weight of 1660kg for something like a 4 cylinder 728i.
The CT6 with a 3.0 Turbo is AWD (?) and weighs 1853kg. A 740i is also an 6 cylinder turbo, but is not available with xDrive (yet)... the RWD one weighs 1800kg (inc. 75kg driver) so 1725kg 'curb'..
if we add on 50kg for xDrive (750i vs. 750iX difference), that's still lighter than the CT6.
I'm assuming the Curb weight figures Cadillac have in their press release don't include driver and cargo, where the EU weights BMW print do.
For such a big car (bigger than a 7-er) the CT6 is light, but based on these figures I wouldn't say it's lighter than a 7-er.