Defining All Undefined Behavior and Leveraging Compiler Transformation APIs

The author seems to be offering a false dichotomy. His strawman argument is to offer platform-specific specified behavior for UB, but then concludes that that would make optimization impossible, operating on a presumption that that behavior must be specified in terms of assembler instructions.

Why not specify behavior in terms that are abstract but simple and sensible for the specific platform, at which point you can continue to optimize on the basis of the as-if principle?