This is publicly available data, published quarterly in the financial statement... I don't recall ever hearing about that kind of a drop.

i presume he means the increase in the traffic acquisition costs like the $12billion (!!) they pay Apple to remain the default search engine in Safari.

i always found this baffling business logic from Google. i think Google is a strong enough brand that a lot of people would switch to it if it wasn't the default. maybe not everyone would switch but not worth $12 billion a year

The quarterly financial statements, and a knowledge of how Google accounting works[1]. Revenue from ads is correlated to CPC (cost per click), ad inventory is correlated to ads on Google sites vs ads on "other" sites, and margin pressure is correlated with operational expenses and traffic acquisition costs. The fuzziest number to model is the ad inventory and relies on estimates of search traffic, ads serviced, and reported revenue and CPC.

[1] I worked there and paid attention at their 'life of a dollar' class they used to give Nooglers. :-)