It's definitely not quite an apples-to-apples comparison. The article mentions the old API used to do things like storing the results in a database which "[...] had grown to store a massive slice of the internet", while the new service appears to be stateless, aside from the cache. Not needing a couple of large database servers alone is going to save a lot of money.

That's not to say Lambda was the wrong choice here, the overall cost is pretty low and it's hard to argue that maintaining their own servers would end up being less than that.