Arnold believed in getting a full stretch and full contraction on every exercise... and I mean FULL stretch. He stood on a wood block and stretched his entire upper back at the bottom of every rep of barbell rows (and T-bar rows, and cable rows), and then straightened his back while bringing the bar all the way up.
FFW to 0:19
You can see that his lower back actually remains fairly stable, while his middle and upper back round down to the bottom.
Many bodybuilders do the same. Here is Serge Nubret doing cable rows:
Mike Quinn did his cable rows like this (if not more stretched).
Lee Haney, Franco Columbu, and Dave Draper all did their barbell rows off a block the way Arnold did, and their backs are legendary.
It also needs to be said that Arnold did not succeed just because "he's special, cuz he's Arnold, so anything he does is magic or drugs, arnold arnold arnold". He was innovative in a lot of ways, out-trained his competitors, and there is a lot to be learned from the guy who has stood the test of time and remained the most aesthetic and awesome bodybuilder of the last 4 decades.