Just randomly throwing it out, but my most likely suspect is Ivy.
The quote goes into detail about how the person would have to have intimate knowledge about the plans for LC, how to bypass the wards, etc. - almost as if they could read and perfectly understand the written wards and plans as they were put up.
Also, Ivy was probably about the right age at that point to read Calvin and Hobbes.
Plus, Harry gave her a name. It's been made pretty clear that that is a Big. Deal.
Presumably, Harry and Bob went over written plans for LC as they were designing and building it - since it's written, Ivy knew about it, and since she is the Archive, she would have known about the flaw. Because she knew what would happen if the flaw weren't fixed, she fixed it.