It's tucked in a typical "help -> about -> credits" in the app just like the hundreds of other libraries. The language documentation never includes documentation on how the functions were built, it's for reading what things do and how to use them.
Indeed. There's no way for users of Wolfram Cloud, for example, to see that information though.
I don't use cloud so I'll take your word there is no such information anywhere but the originally linked post is the author saying they don't think it's used in Cloud anyways.
I'm the poster. I think you misunderstood the followup post -- the same library is definitely used in the Cloud (same as the standalone Mathematica).
Appreciate the correction, you'd be right - I mistakenly read it as cloud vs local not normal vs interval.