It seems to me, that it might be more complicated than that…
For example take a specific file from a repo: discourse/CONTRIBUTING.md at master · discourse/discourse · GitHub The iframely link says no embed code: https://iframely.com/embed/https%3A%2F%2Fgithub.com%2Fdiscourse%2Fdiscourse%2Fblob%2Fmaster%2FCONTRIBUTING.md but then Discourse itself / onebox happily embeds it:
https://github.com/discourse/discourse/blob/master/CONTRIBUTING.md
If iframely was correct, then neither the repo link, nor the file link would work. But one does and the other doesn’t…