"Exploiting undefined behavior" occurs when a simple semantics (however one defines "simple") results in behavior A, but the compiler chooses behavior B instead based on the actual, more complex, language semantics. The code snippet in question passes that test. If I flip the declaration order of i and arr, then I get this [1] at -O0 (the "simple" semantics):

        push    rbp
        mov     rbp, rsp
        mov     dword ptr [rbp - 4], 0
        mov     dword ptr [rbp - 4], 5
        mov     eax, dword ptr [rbp - 4]
        pop     rbp
        ret

Which indeed returns 5. But at -O2 clang optimizes it to this:

        xor     eax, eax
        ret

Which returns 0. So the simple semantics produces one result, and the complex semantics produces another. Hence, it's exploiting undefined behavior.

[1]: https://godbolt.org/z/df4dhzT5a