I checked the footnote. It describes a case where invoking UB (transmuting "3" to a bool) can cause dead code to become live. But transmuting "3" to a bool is

I checked the footnote. It describes a case where invoking UB (transmuting "3" to a bool) can cause dead code to become live. But transmuting "3" to a bool is itself UB. So this fails to prove that execution becomes undefined without reaching a line containing UB.