I’m sorry for the delay.
Yes, with the current version you have to use the default WP REST API for the “hp_listing” post type. The listing submission form is basically the listing update form, because the listing auto-draft is already created if you click the Add Listing button.
Attributes are implemented depending on their type, e.g. text/number attributes are implemented as post meta fields with “hp_” prefix. Selectable attributes are implemented as custom taxonomies with “hp_listing_” prefix, the listing category is a custom taxonomy of “hp_listing_category” name.
This way you can still set all the values for attributes on WP level.