Google's Indexing API supports two content types: JobPosting and BroadcastEvent. This reads the JSON-LD on a page, checks the properties Google requires, and tells you whether the page is one of them.
Google's Indexing API accepts any URL you send it. It does not check whether the page is a job posting before returning success, and for anything that is not one of the two supported types, the notification is ignored.
A tool can pipe your entire sitemap through that API, show you a success count for every URL, and have changed nothing. The response says "notification received". It has never said "page indexed".
The only way to submit through this API and mean it is to check first, and refuse the pages that do not qualify. That is what this page does, and it is why we can tell you it matters.
That the page carries valid markup for a type the API is built for, so a submission is a real signal rather than a discarded one.
It does not promise a rich result or a fast index. Google applies its own content policies on top of valid markup, and a posting that is expired, duplicated, or behind a login can be perfectly marked up and still rejected.