I believe the result of the following test case should be false, not true:

   <test-case name="Castable-ListType-9" covers-30="cast-to-list">
       <description>Test castability of empty sequence to list type.</description>
       <created by="Tim Mills" on="2016-04-11"/>
       <environment ref="union-List-defined-Types"/>
       <dependency type="spec" value="XQ30+"/>
       <dependency type="feature" value="schemaImport"/>
       <test><![CDATA[
       import schema namespace s="http://www.w3.org/XQueryTest/unionListDefined";
        () castable as s:intListType1]]>
       </test>
       <result><assert-true /></result>
   </test-case>

The spec for castable to list says that the input must be a (single) string or untypedAtomic; it also says that an empty sequence is OK provided the type is written as T?.