Well the first part wasn't clear from your post at all, you mentioned both Pele and di Stefano being weaker and went on to use the defenses in the 50's to dismiss their achievements. Anyway, none of them was just a goalscorer. Di Stefano was arguably the most complete player of all time, he was a tough midfielder, a playmaker and a goalscorer in one, he defended, assisted and scored. I don't think I've ever heard anyone saying he's one of the greatest because of his goal record.
And regarding Pele, no one really cares that much about his pure goalscoring record either, well no one besides himself. What lifted him to that greatest of all time level was the transformation late in his career towards a playmaker with the role defining performances as a number 10 in the greatest nationalteam of all time. While he indeed played most of his games in the Sao Paulo state championship, it wasn't just a regional league. There were 2 or 3 important state championships in Brazil and many great Southamerican players moved to those leagues, which meant they were actually on a very high level and not just full of players from that region. Then there's the fact, that he also scored lots of goals in continental and intercontinental competitions. For example, Pele scored 6 goals in the 2 games of the Intercontinental Cup against Benfica in '62 and 4 more in the games against Milan in '63. Pele made those exhausting trips to Europe and showed the best European teams on their homeground how far above their level he is.
Regarding defenses in the 60's. The goals per game average last season in La Liga was 2.74 as far as I know, I google the website that has those numbers later or you can just look yourself. It's more or less the same as it was in the 60's if you look at
@Skorenzy 's post (maybe he can provide stats for the past 5 years as well, so that we have a comparison). How do you explain that if defending was such a huge joke back then?