Displacing might not be the actual word. You could, for example, say that the digital ad market will double in 5 years, and Snapchat will capture all of that additional revenue, becoming bigger than FB and Google combined.
It wouldn't be displacing them per se, they'd still sell the same $ to the same companies.
Will that happen? My guess would be on no. I don't think there's room for a 3rd large player in the digital ads world, Twitter tried to be it, but failed. But hey, maybe Snap is able to do it (but I doubt it).
It displaces them in the relevant metric (ad penetration), so I think it still fits fine in this context. It would be different if the original commentator was arguing that Snap is going to displace FB/Google's current revenue.
They are dealing with economy of scale. If facebook+google have 90% ad penetration then snap must displace facebook.