Marxist literary criticism tends to look at a piece of literature as a piece of evidence that tells us about the times in which they are written. So they would ask what it is about Orwell's time and place and especially his class that made him write the book in the way he did.
I would think that a Marxist would say that the book does not reflect the reality of class struggle. There is no sign of class conflict based on economic class -- you do not have the workers struggling against the owners of the factories.
Instead, the book is only concerned with bourgeois ideals of love and freedom. To me, a Marxist would say that these concerns are irrelevant. They would say that Orwell was clearly not concerned with the more important things in life.