Flakey salt is not the same as kosher salt. I just flipped through my four most used cookbooks and they all say "salt measurements are for kosher salt" in the preface somewhere. Three of them specify diamond crystal.
I've also got a coffee table book written in 1999 that says the same.
Agreed. Kosher salt is mainly not flaky, it just has slightly larger granules than table salt. I think the poster above might be conflating some of the flakier salts you can get which might also be kosher with standard kosher salt.
I've also got a coffee table book written in 1999 that says the same.