I think you're conflating quite a few different people there. I mean, there probably are some people who simultaneously hold all those views, but most don't. For example many people doing social studies of science do find science interesting, which is partly why they spend so much time studying people who follow that methodology— they just don't think it's a methodology that is independent of the human culture that it arose from, and are particularly interested in how its social practices developed (even if the scientists themselves don't like to see what they do as "social practices"). Bruno Latour goes in this category, as something of a 'science-o-philic' social constructionist (although he's also a "soft" constructionist, because he views both humans and non-human entities, like lab apparatus and materials, as jointly doing the construction, vs. it being a purely cultural phenomenon). And generally (most) social constructionists and (most) Marxists don't see eye-to-eye, precisely because most Marxists do think there is a scientific, objective method (historical materialism) for analyzing social relations, history, economies, etc., and orthodox Marxists even tend to believe objective factors ultimately determine culture rather than the reverse.

The spesific dude/dudette who invented the "nationalism is modern social construct and pretty false" is in my understanding quite left leaning. People who say this stuff out loud in real life (all three of them) are very left leaning. Being completely opposed to nationalism makes sense in very few instances, and comitern sympathies are one.