"You know that the Ariane 5 rocket crashed (and could have killed people!) because of an int overflow? What if people die because you decided the C standard allows you to optimize away other people's security checks?"
Sounds reasonable to me. As a security consultant, it's important for his work and his moral duty. Think of the children ;-)
That integer overflow is undefined in the standard also doesn't make it go away. Integers overflow everywhere. It simply means that the details of it are not part of the standard. The new behavior he's complaining about is not any more standards-conforming than the previous one.
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=30475