roids aside, arnold was a champ because he simply was the most dedicated bodybuilder at the time.
most bodybuilders nowdays would not have achieved success in his days. back then, your main source of protein was solid food, and any protein shakes that existed tasted like liquid turd. for him to maintain a championship diet, it took 100x more willpower then it takes today. if you can have that kind of willpower, you can accomplish anything, even become a governor. nowdays you have restaurants that cater to bodybuilders, protein shakes that taste like chocolate covered strawberries, and all sorts of sh!t.
also, arnold lived a very active lifestyle, even outside of the gym. he worked a ton of blue collar jobs which required physical labor (i remember reading about him working as a brick layer in his bio). the guy was a very active person, and as a result his body was able to adapt to his lifestyle. hence him being able to handle that ridiculous amount of sets.
while i do think that any average person would overtrain on his routine in a matter of 2 weeks, i do think that if they had his dedication, they could push their body to eventually adapt to it (it would take them years however just like it took arnold). and i do think that his splits benefit roid users more then any other routine. i remember reading ronnie coleman's site way back before and some of his splits were almost identical in frequency to arnold's.