Sorry but this review is factually wrong. Looking at the lootlist I made for it, you certainly don't need to find every gem available in the mission to fill the gems quota, and you don't even need to find any secrets to reach it. You also mention that the key in the sewers is not found where the readable says, but it literally is found where the readable says it is, and in a logical location to boot.
Also, if I remember correctly you do need to find a secret compartment in Thief 2's Runnin...