The context is that it's a footnote. The footnote is referenced in this paragraph:

A postfix expression followed by the . operator and an identifier designates a member of a structure or union object. The value is that of the named member (106), and is an lvalue if the first expression is an lvalue. If the first expression has qualified type, the result has the so-qualified version of the type of the designated member.

106) If the member used to read the contents of a union object is not the same as the member last used to store a value in the object the appropriate part of the object representation of the value is reinterpreted as an object representation in the new type as described in 6.2.6 (a process sometimes called type punning). This might be a non-value representation.

In that same document, union type punning is explicitly listed under Annex J.1, Unspecified Behavior:

(11) The values of bytes that correspond to union members other than the one last stored into (6.2.6.1).

The standard is extremely clear and explicit that it's not undefined behavior.