"All enzymes are not proteins". Which of the following justifies this statement?

Enzymes are protein catalysts that increase the velocity of a chemical reaction and are consumed during the reaction they catalyse. Not all enzymes are proteins, some types of RNA can act like enzymes, usually catalyzing the cleavage and synthesis of phosphodiester bonds. Ribozymes are RNA molecules which catalyze cutting of nascent mRNA or primary transcript. Ref: Lippincott's Biochemistry, 2nd Edition, Pages 48, 53 ; Textbook of Biochemistry By D M Vasudevan, 3rd Edition, Pages 30, 45