But not on specific compilers with specific settings on specific machines like were used for the benchmarks results. Execution time is also undefined by the C standard.
"Undefined behavior" is a specific term used by the C and C++ standards. What you're describing sounds more like "unspecified behavior", something entirely different.
Undefined behavior allows for aggressive optimizations where the compiler assumes the integer overflow can never happen, leading to such wonderful bugs as this:
Some compilers may offer specific settings, such as fwrapv, which give you a way to define the behavior of integer overflow. (Although last I read fwrapv was broken.) I see no mention of any such specific setting in the article. If you've spotted one I've missed, please call it out more specifically.