Hey @Polarts ,
welcome to the community!
Your query should actually only return items and no subitems. Maybe the subitems have the same name and your are mistaken them as subitems.
Subitems are basicly column values of an item in the given board. So they are not counted as items, hence they don’t appear in that query.
To visualize what I mean mean here a thread:
Hope that helped you and don’t hesitate to ask.
Always welcome 
Greetings