About 1): I think the point was that CL can only sometimes enforce invariants at compile time. If the compiler can't prove that something is always true or always false, it will simply emit code to do a runtime check.
But as a general principal, you can't reliably or consistently do this in a dynamic language in any sort of guaranteed fashion, which was implicit in the original statement being refuted.
Even with a runtime check, you still CAN pass in a bad value, and the program will just fail, either locally or globally.
(Not to mention that the paragraph immediately following the refuted statement starts with 'Now I am fully aware that “the compiler can optimize this away”, at least in some cases, but to achieve this requires one of two things (apart from unreachable levels of ingenuity that can easily, and more profitably, be expressed by the programmer in the first place)...')
Even with a runtime check, you still CAN pass in a bad value, and the program will just fail, either locally or globally.
(Not to mention that the paragraph immediately following the refuted statement starts with 'Now I am fully aware that “the compiler can optimize this away”, at least in some cases, but to achieve this requires one of two things (apart from unreachable levels of ingenuity that can easily, and more profitably, be expressed by the programmer in the first place)...')