Complementary. Silverblue’s immutability may be aimed purely at stability and therefore may not have strong guarantees, but if it did then the next attack vector would be from root to kernel (in memory), and that’s where lockdown comes in; it’s designed to prevent attacking a running kernel as root.
It basically doesn't try to fix the building blocks with all their legacy...instead it simply makes the entire operating system immutable.
I wonder what are the pros and cons of each approach. Or are they complementary.