The way this works (or rather, worked) is that you first need to "go get" a module and then it gets picked up by the pkg.go.dev system. This works like that for every site, and was kind weird and confusing, although as you mentioned this was improved on recently. Either way: not related to that module being on sr.ht; that's just coincidental.
That is why they decided to turn it into an internal/source package inside pkgsite first to have test cases for a later go-source-2. If your site does not show source links on pkg.go.dev then add a comment to issue 40477 above and an exception will be added.
Note1: Maybe mention this on your website because that took a little to find. It does not fit into the narrative though.
Note2: Requiring frontends to offer meta tags is the only way to make code discoverable with the url alone. cgit not offering this is on cgit and not on Go/pkgsite (haven't checked whether they do)
The way this works (or rather, worked) is that you first need to "go get" a module and then it gets picked up by the pkg.go.dev system. This works like that for every site, and was kind weird and confusing, although as you mentioned this was improved on recently. Either way: not related to that module being on sr.ht; that's just coincidental.