> A program with undefined behavior is trivially not compliant with the specification.

Undefined behavior means doing something the compiler spec does not specify, therefore the compiler is free to do anything. There are plenty of programs that exploit the programmer's knowledge of a particular compiler's actual, unspecified behavior as an optimization.

One example is using an int variable after an overflow. Post-overflow, the int's value is undefined behavior, but in practice it is predictable for a given compiler and platform. It is possible to write a program that implements its own program spec correctly, while exploiting out-of-spec behavior of its compiler under the hood.

Much like how it's possible to wrap unsafe Rust code with a safe interface.