Not sure how long the subreddit has existed but I always assumed it was inspired by the episode of The Office where Michael asks Oscar to explain what a surplus is like he's 5. The broader usage on social media and the ELI5 shorthand are definitely the result of the subreddit though.
This article is definitely abusing the terminology, because I wouldn't expect even tech-knowledgeable people to know what Nostr is or does.
I remember when the sub was first created, I think there was an understanding it didn’t actually mean you were explaining to five year olds. Hard to recall exactly when it was created though, so definitely awhile ago.