A lot of datasets looking at Congress over time smooth over small nuances. They look at the bi-annual election results and assume that’s what holds until the next election. The problem for answering a question like this is that Congress changes between elections. The Democrat majority includes Arlan Spectre, who defected. Al Franken, whose election was contested and not resolved until months into his term. Ted Kennedy and Robert Byrd were ancient and in the process of dying (perfectly fair to blame the party for letting them cling to office too long. Some things never change). The article doesn’t mention it, but I think that 60 senators includes one or two independents who caucused with the Dems too, like Bernie Sanders.