Arnold and his contemporaries frequently employed cycle strategies which included tapering off, pyramiding, etc. to account for not having anti-e's (With the inclusion of anti-e's, these strategies are no longer necessary and are out-dated) Also, most of us, particularly mesomorphs, do not have tissue sensitivity to gyno (breast growth). Back then, you were usually introduced to AS if you were already a bodybuilder with promise or a top athlete, so most of those who may have been prone to gyno were never introduced like they are today. I doubt Arnold and many of the guys back then spent a lot of time worried about getting gyno.
Gyno certainly occured back then, but as it is today to some degree, Gynecomastia is a condition that is not spoken of openly. There was much less "recreational" AS use back then, and therefore, fewer guys using that had no idea what they were doing, unlike what is the case today.