LT makes it clear

Bourgeois poetry, of course, does not exist, because poetry is a free art and not a service to class.*

*I received from an experienced and well-read journalist a thundering letter, proving the class character of literature. My correspondent took the sarcastic sentence literally. I am afraid that this might happen with others. There are not too many attentive readers in the world. I am therefore driving this note home with this inscription: "Attention! This is irony!"

from Literature and Revolution by Leon Trotsky (1924)