Hello David,
We are using WordPress built-in functions to delete the posts.
So it will remove all the meta data like post meta, taxonomy association, revisions etc.
The only things that will not be deleted is the attachments since it is possible that the attachment may be used with some other posts. This is also the default behaviour in WordPress.
We also provide another add-on called Bulk Delete Attachments that can be used to delete all attachments that are not attached to any post.