I saw several dozen comments today about the lack of the notification LED on the 5X based on its absence from the specs, so I decided to reach out to +Dave Burke
(Google's VP of Engineering) to figure this out once and for all.
Thanks to Dave, we got the full picture of what's going on here. Read on to find out.
Tl;dr: Both phones have true notification LEDs (unlike N6), but because Ambient Display is used for notifications, they aren't going to be used for actual notifications, but rather accessible to apps like Light Flow, etc.
Now go ahead and correct this misconception everywhere you can see and point them to the article.