Two inconsistent clauses

Before the Spanish conquest of Peru, the coca plant was reserved for Inca royalty, who used it in mystical rituals and celebrations. By the sixteenth century, when Spanish explorers first began arriving in South America. the native inhabitants had a 5,000-year history of chewing coca leaves to fight fatigue and hunger and to increase endurance.
[Source: Reading for Results Ninth Edition by Laraine Flemming]
I think two underlined clauses are inconsistent with each other.
I'd like to know what you think about this.