You can read and write HA entities via HTTP API so I think you could have done this with nothing but HA, but either way it's a fun project :)
One of my more fun HA project was scraping my snow removal service's tractor tracking API and passing the result for the closest tractor into HA so I can be notified (by voice with Alexa and/or push notification) when there's one nearby in case I need to move the car.
Do you mean outside of the API that I talk about in the article? If I exposed the HA API via the frontend, users would be able to just take the bearer token and do whatever they wanted with it (even deleting the entities used for the doors). That's why the express server is sitting in the middle to only expose the relevant information (proxying w/ the bearer token, which the end users have no access to)
One of my more fun HA project was scraping my snow removal service's tractor tracking API and passing the result for the closest tractor into HA so I can be notified (by voice with Alexa and/or push notification) when there's one nearby in case I need to move the car.