IIRC xbox discs are in fact read backwards, the reason a PC drive cant read it is because it reads the outer section (which is a dvd video telling you its an xbox game, and instruction you to put it in an xbox to play it) and then its a hard "end of disc" marker so it doesnt read any farther
xboxs read from the inside and see the game before the dvd-video (then later hit the end of disc marker and never get to the video portion)
for a backup, it only has the one region, the game, so it reads (maybe the inside out, maybe from the outside in, who knows) and eventually find the default.xbe, and runs it
if its an unmodded xbox one of two things happens, it hits the xbox and tries to run it, if its signed it runs it, but the media check within the xbe prevents the game from playing, if it has been media patched its signature is no longer valid, so the stock bios doesnt play it
for xbox1 modding, the hacked bios ignores the lack of proper signature, for a 360 the firmware patch counterfeits the media check and reports a proper, retail pressed DVD instead of a dvd-r
for PS2 the media check is not the in the executable, but in the ps2 firmware itself, thats why burned movies wouldnt play on old ps2s (i think they later updated it so it would play dvd-video from a burned disc, but not ps2 games)