get-posts-stateless - how to get 1st level comments?

[pangia]

There is a 'FetchSubComments' flag to set (which does not seem to work, maybe it is dependent on other flags?).

I see no top level comments option for this api call. I need to get all of 1st level comments for each post without having to call the 'get-single-post' api. is this possible?