Let me try to justify that astounding statement. The book was one a deep theme, rich with invention, and daring in its mystery.
Lindsay used the idea of a voyage to another world as a metaphor for the spiritual confrontation of all man's noblest and darkest impulses. It is a story about the meaning of life and death. Something of a norse pagan in his philosophy (unfortunately) led the author to a resolution that is dark and unchristian: a worship of the stoical Self as the source of glory, a defiance of the created universe and its pleasures, and pain elevated to the only true sign leading one to divine fire.
In terms of his sheer inventiveness, however, no modern science fiction writer matches him. The first thing the traveler Maskull sees on the wild, huge world of Tormance are two new primary colors, which the author can only describe indirectly.
A day later the mutation-forming sun Branchspell grants new sense impression, these serving the will rather than the senses. Later the organs mutate again, telling him of the inner natures of beings, or allowing him to see duties, or to absorb souls.
Later he hears a form of music that manifests as sensations at first painful, but then elevated to a sublime and ineffable spiritual force.
The author is trying something as hard as describing the fourth dimension.
Later the traveler meets a human neither male nor female nor hermaphrodite, but a third positive sex, inventing the new pronouns, 'ae' and 'aer' to refer to this being.
Throughout this richness of invention, new colors, new senses, new sensations, new sexes, the author also maintains what can be one of the world's only spiritual detective stories: the world of Tormance is riven between a duel of two gods, but the divinity is disguised, and each fantastic new creature the traveler meets has a different tale of the truth of things. The rich symbolism in which this pilgrim's progress is veiled gives the only clues to the recondite answers.
The sense of wonder for which science fiction authors try is limited by their materialistic outlook. For a sense of the truly fantastic, oriental wonders as dazzling and strange as life itself, one needs to move into something like the spiritual hallucination and symphony of Dunsany.
I will not call it a good science fiction book, and I am not sure if it would be classed, these days, as fantasy. It is a work that stands alone.