In Minecraft you can dig blocks.

When you click to dig a block, it just calls the RPC method CPacketPlayerDigging with the coordinates of the block.

The server replies "Block of type ABC is too far".

So, of course, you now know that at (X,Y,Z) you have a block of type "ABC".

This RPC has no rate-limiting, so yes, you can know the whole map.

The fact that you can dig any block on the map, seems like a shortcut taken by developers of the server to save time or resources.

I'd even say it's almost intended behaviour.

Tech details: https://2b2t.miraheze.org/wiki/Nocom