I think his point is it possible to quantify what the NSA does and not reveal anything classified. Similar to how you can quantify how good a football defense is even if you don't know every play, or anything about their strategies, who is on there team, how many players were on the field.

Are there many/any government agencies that quantify things to validate their existence?

Schools have tests. I get a newsletter from my local government a few times a year that says how they are spending my money, especially if they are pushing for a levy. Any program that is in jeopardy of losing funding publishing some sort of "this is why you should keep giving us money" like NASA or an arts program. For example: https://www.nasa.gov/news/budget/index.html

I think the point is, it's your/our fucking money, and they should be.