They have APIs that other developers can use to build things, but not for embedding content into the pages itself (I haven't searched so much about Reddit but I

They have APIs that other developers can use to build things, but not for embedding content into the pages itself (I haven't searched so much about Reddit but I have about HN). So basically these libraries are for pulling data, not for pulling functionality. In fact I am using one of them, Algolia on the back-end to pull the data. For truly embed it we'd need for HN to provide this functionality with some Javascript library/iframe/other, which they currently don't do.