The guarantee is not against a malicious agent, but a reasonable programmer who may make mistakes.

A function may not reliably cast a pointer-to-const-T to a pointer-to-T because T may actually be immutable, in which case modifying it would be UB. Like all casts, const_cast exists as an escape hatch in the type system for when you know that what you are doing is safe, even if the compiler can't prove it.