How to use template part?

Hello,
I just saw the JS for WP Conf Talk from @SantosGuillamot and was very excited about the possibility to load Gutenberg template-parts into Frontity.

So I tried to do the same… But it won’t work for me because we no longer have access to this api route without authenticating but I would still like to use this

https://rockusupport.com/wp-json/wp/v2/template-parts