Being somewhat experienced at game networking and writing server code with this type of exploit(e.g. sending a packet to do something at the other end of the map and processing the result) in mind, this would be something I either would have checked for in the first place if performance would have allowed for it, or would have kept in the back of my mind in case I hear about a possible exploit that takes advantage of it.
Another reason might be that this only does anything if there are many players on the server, and you hit a region loaded by someone else. Might be difficult to think of that. You'd never come across it if testing in singleplayer.
But in essence I agree, it's surprising that no one else found and reported this years ago!
-Tell you what block is placed there, if the chunk is currently loaded
-Or ignore you, if the chunk is not currently loaded.
2b2t would immediately reveal whether or not any chunk coordinate that you ask about is currently loaded. If it is currently loaded, it tells you which specific block is there at the coordinate you asked about."
This allowed the hackers to find every player's coordinates and where their bases were.