Oh looks like they are outsourcing it to embed.ly.
Digging into it, appears that using the post ID can be used to construct a working iFrame after all.
Unless @techAPJ wants to take it, I’d say pr-welcome
<iframe src="https://old.reddit.com/mediaembed/gdyrsg" width="690" height="388">
[AN IFRAME THAT WORKS AND AUTOPLAYS THE VIDEO]