Concretely, here, the UB-exploitation in question in this case is assuming that the "this" pointer in C++ is aligned and non-null, meaning it's a pervasive annotation throughout C++ codebases, not an edge-case.

Relying on LTO to "discover" this annotation through interprocedural analysis -- based on my experience of looking at LTO in practice -- will not be as comprehensive, and even when it works it accomplishes its task in an achingly-slow and expensive way.

This is a real devil-is-in-the-details case.