to run an exploit, you usually need to overflow the buffer size defined by the developer to write your own code in another part of the memory which will be executed by the console without knowing it's not the correct function anymore.
there's no way to overflow the MBR verification process, as they are reading the sector 0 themselves, using their own defined size.
they create a buffer with a fixed size (usually of the size of one sector, so either 512byte, or 4k).
they Read the first sector's data and store it into the buffer up to the specified size (it will not overflow, they specifically tell how many byte to read/write, the console will not continue to write more data outside of the buffer)
they check the buffer's bytes 511 and 512 to see if they match what they expect. (you cant code anything in two byte, it's only 2 characters, and it's doing a comparison, not executing any part of the MBR's data)