Rubinstein was the true prophet of the composers he played. His Chopin is but a small fraction of his musicianship. He also played Brahms, Schumann, Liszt, Prokofiev, Villa-Lobos, Albeniz, Debussy, Beethoven . . . everything to full conviction and authority. The only snags were found in some of his Chopin Preludes - which he never recorded for stereophonic anyway.
The grand golden tone, the natural flow of music and temperament, among many others, were the hallmarks of Rubinstein the artist.
My opinion - when Rubinstein played all other pianists could've well packed up and gone home. Argerich and Horowitz are by leaps and bounds no match for Rubinstein in musical terms, although they have phenomenal technique.
Branding Rubinstein as the "Chopin Player" is a gross understatement. He was an all-round artist, convincing in everything he did.