The US has been union unfriendly since the 19th century when union leaders were regularly assassinated, and in the 20th century when striking workers were aerially bombarded and killed.
Things had to change after the Great Depression though, the Roosevelt government had to actually make amends with and negotiate with labor unions, since the alternative was widespread economically-induced unrest which might possibly lead to communist or fascist uprisings. I think the parent comment was talking about that particular era, and commenting on how Reagan dismantled that whole deal.
mind you this was historically the case nearly everywhere in Wurope too. This only really changed drastically after world war 1, which saw the fall of many empires (german, ottoman, astro-hungarian and russian) which resulted in a lot of political instability. Trade unions and other labour organisations where able to demand representative power in a turmoil time when many new countries where created. (having a nationwide strike or even a revolution was a really bad thing for the (newly established) establishment).
This got further cemented when the USSR was formed, and it actively supported many left winged militant organisations in europe. This was used as leverage by trade unionists and social-democrats to demand further rights (or else the militant forces will get into power and things will end much worse for the establishment).
This dynamic only really changed right before world war 2, when stalin (and other USSR leaders) focused on a policy of "socialism in one country" instead of a the dogma of a worldwide workers revolution.