The Trello + Zapier connector is actually one of the oldest connectors to Zapier. Zapier's team actually initially built it themselves and later handed it off to Asana to manage.
While it's a pretty solid connector for this reason, it doesn't have all of the endpoints of the Asana API baked in, so you're often going to be referencing the actual Asana dev API when trying to do less common things.
If you'd like to do API actions that are not yet in Asana's Zapier connector, you must use Asana's native API along with the Webhook functionality of Zapier.
(This is a more technical feature that requires API knowledge)