There are few ways the word Horcrux could come out of Harry's mouth that I would not take as evidence for Harry doing outside research. It would take non-trivial circumlocutions to indicate that Harry doesn't know what the English word Horcrux means. Such circumlocutions appear in the text. The best reason I can think of for those circumlocutions to appear in the text is to prevent people like me from inferring too much from Harry's vocabulary choice. There are alternatives explanations, like hinting that Salazar invented Horcruxes, but they seem less likely to me.