I think the author is discussing the movement of art into abstraction. From Baroque to Cubism art became increasingly abstract. It went from representing “real” people and things to emotions. And in cubism and surrealism, an ideal the artists strove for was to paint emotions as purley as possible to the point of attempting to express directly from the subconscious. And yes, the artists wanted their works to be suggestive but not exact so an amount could be open to interpretation. And with that I personally think they got a little too carried away (I’m not super into cubism or abstract art - but that’s just me personally).