A noindex you cannot see in the HTML

View the source all you want. This page's noindex is in the HTTP headers.

Headers can control indexing too

The X-Robots-Tag response header does what a robots meta tag does, but from the HTTP layer instead of the page. It is handy for files like PDFs and for setting rules at the server or CDN level. The catch is that it is invisible if you only look at the rendered HTML, which is how it can hide for months.

Why a tool has to check both

A crawler that reads only the HTML will report this page as indexable, because nothing in the markup says otherwise. Catching a header-level rule means reading the response headers on every request. This page exists to check that an audit does that, instead of trusting the HTML.