You probably misunderstood. C can represent any program's semantics, since it's Turing-complete (modulo finite memory). C can't encode the lifetimes Rust uses, but those get erased during compilation to MIR. This takes MIR from rustc (where borrow checking has been completed and lifetime annotations erased) and outputs C with the same semantics. LLVM doesn't use tokens not produceable by C, but rustc does.
C still has lifetime rules. It just has no syntax for them. What people are bitten with is that they violate the lifetime rules, but their compiler doesn't tell them.
I think it's a reference to certain optimizations possible due to aliasing rules in Rust that are not possible (or maybe only "not straight forward", I'm not sure) in C. So a transpiled program while keeping its semantics might not still compile to equally optimized assembly.
IIRC C can do the same things with correct usage of `restrict`, but that's extremely difficult by hand. So difficult that LLVM's `restrict` support was very buggy when Rust first started using the capabilities. Those bugs got fixed, but it's still impractical to use in handwritten C.
