The notion that Sequoyah was a monolingual Cherokee is dubious. He had a European father (though he was raised with his mother) and worked as a trader and served in the U.S. Army. His cousin, to whom he presented his syllabary, was also half European, “George Lowery.” He had extensive contact with Europeans. Moreover, his syllabary includes adaptations of Latin, Greek, and Cyrillic letters. Part of the story is that he copied some character shapes from his wife’s family’s Bible. (Presumably they could read English if they had a Bible.) He was obviously exposed to a variety of European writing. He completed his syllabary in 1821, many years after his military service. It seems highly unlikely that someone who was so linguistically gifted to be able to invent a syllabary would not have picked up some familiarity with spoken and written English through that exposure.

This article does a good job of reviewing the conflicting narratives of his history: https://www.jstor.org/stable/26467045. It’s all very uncertain, and there’s a lot of mythology.