It's a material relation, which is quite different than a social construct.
I think this is a common point of confusing these days and it's why so many on the left have been sucked in by identity politics.
Here is an example of the difference:
Race is a social construct. There can, on the basis of that construct, be inequalities. But there is no inherent reason that the relation between say, Europeans and Africans, needs to be unequal, or the direction of that inequality. In 500 years, it could be gone, or Europeans could be an oppressed race, or something else. It's concept is fundamentally an abstraction with arbitrary boundaries.
When you look at class, you are looking at a concrete relation. A member of the working class, who can only sell his labour, is in a particular relation to the person who owns the factory. You could swap the individuals around, and the factory owner, because he is the person who owns the productive capital, will still have the same power relation to the person who he employs who has no capital or means to work unless someone else employs him.
Or look at the landowner vs the peasant who labours on his land. The landowner has something that can produce. If he has no one to work for him, he can at the least work on the land himself. The farm worker has nothing, he has only his own body, if no one will hire him, he will starve. So he is in a position with a concrete disadvantage.
That's not a construct, it's about the nature of the physical word.