Shape change comes in a variety of forms, including druid shape change, lycanthrope shape change, and the shape change spell. They vary in nature, and are not the same as polymorph (e.g. polymorph, polymorph self, monster polymorph).

See Death.

Being shape changed into a monster does not bypass the “This does not apply to characters of any sort” line on DMG 75. Shape changed characters must have some external means of striking such creatures, such as a magic weapon, spell, or magic item that specifically grants the shape-changed possessor attacks that can bypass such defenses.

We are not 100% sold on this ruling yet, though the text does appear very clear. If we find sources that suggest this ruling is bad, we will change it.