The "Class.relation.append" syntax is a shortcut for creating VALUE constraints. This syntax treats classes as if they were instances, and only support VALUE constraints currently.
To create SOME constraint, you need the more complex syntax :
You can use Class.equivalent_to instead of Class.is_a for an equivalence.
You may need to prefix the relation by the ontology (for example: onto.has_part), depending whether you created it locally or you load it from an ontology.