I think I know why.
In recent years, EA has started to blur the boundary between an element type (which is definitively described in the otObjectType enum) and a stereotype.
So, when EA lists what is describes as ‘Element Types’, in many places, that’s a mixture of true element types – like ‘Class’ – and common stereotypes – like <>Class.
There’s no obvious pattern to when EA does this, so eaDocX has no choice but to take a strict view of element types vs. stereotypes.
Element Types are the values which appear in the Object_Type column of t_object, and no others are allowed.
Stereotypes are everything else.
So, to print <>Class elements, create a Profile for that stereotype, and I’m confident that all will be joy and happiness.