Black slaves were neither workers nor immigrants, as also stated in the textbook. Being forcibly kidnapped, sold into slavery and shipped in chains over the ocean in appalling conditions is not "immigration". But then some US Right Wingers preach that Blacks were better off as slaves than free.
Yet these are the people rewriting US history books through the publishing Achilles heel of Texas, due to its large school population. They've seized control of the State board that selects school books, easy to do in that predominately Red State, and book publishers make all their books throughout the US comply to Texas Right Wing ideology for economy of printing & marketing costs.
A clever tactic by the Right to indoctrinate US youth with their beliefs. It's the same reason they've underfunded secular public schools while diverting the money to conservative religious charter schools.