What clause(s) support the claim that that example is UB? At least at first glance it looks pretty similar to the sometimes-recommended way to perform safe type-punning in C, and the only way to "directly" invoke UB via memcpy (passing pointers to overlapping objects) isn't relevant here.

The only suspicious part to me is the size/number of bytes to copy, but I'm not sure that's outright UB?