In this case, "Zero-JS" seems to mean that you write short bits of JavaScript that look like a domain-specific language, and you include a relatively small JavaScript file that translates certain server-sent events into DOM updates. The documentation for Datastar is less assertive; for example:
> The value of the data-text attribute is a Datastar expression that is evaluated, meaning that we can use JavaScript in it.
(from
https://data-star.dev/guide/reactive_signals)
I personally would call this a minimal-JS approach to an SPA. The use of Nim as a server language and YottaDB as a storage engine seem incidental, except that the overall page is advertising for the database.