Internet slang from the early aughts(2000s) indicating someone replied with an answer shortly before your post, without you realizing it.
I think it fell out of usage in more recent years now that you can see posts/replies quicker now that most sites don't require a page refresh to see new comments. Also maybe due to the acceleration of eternal summer/september when big social media sites started gaining traction.https://www.urbandictionary.com/define.php?term=ninja%27d
I don't think I've ever seen that used before, I would have been exposed to it too. Maybe I just forgot anyone said it, definitely haven't seen it in 10 years or more.
Time sure has flown by.