But we were talking about a car that is very overweight even compared to much larger vehicles, which is not good.
Lightweight materials are usually stronger than plain steel so weight reduction usually comes with a rigidity bonus. Their biggest drawback is high maintenance cost, and Lexus is known for low cost of ownership. This might be the biggest reason Lexus is not overly commited to weight reduction. But being overweight is another issue.
Yes, however we don't know the full scientific specs of all the competitors compared to the LC. For example, we do know that the LC is even more rigid than the LFA, and that is an extremely rigid car. How does that compare to competitors? What about door thicknesses, or body thicknesses, and other such parameters versus the competitors? We just don't know. Also in traditional Toyota/Lexus fashion, some mechanical parts could be overengineered for reliability/quality/durability vs the competition, thus added weight.
Also stronger is relative, and a very broad term. Some lightweight materials have strong stiffness and have high tensile strength yes, but some drawbacks do exist. Some lightweight materials are weak in torsional rigidity. Some lightweight materials will break, snap, or crack very suddenly, vs other materials that simply bend or flex under such conditions. Vehicles experience a lot of twisting and shearing forces, not to mention huge amounts of vibration and wave forces during the lifetime of a vehicle. Some lightweight materials don't handle torsion, vibration, or wave forces very well.
So there are too many details we just don't know with the LC vs the competition regarding weight. If I had to guess though, the LC is overweight due to being overengineered for long-term quality/reliability/durability. This is even considering the LC has curious oddities and omissions like the lack of high-end power seats with large ranges of adjustability.