Picking up something established is adopting it. I would say he and Elvis adopted and then popularized it for mainstream white audiences.

It's appropriate to credit them among the first ever "rockstars" for this reason. But I would never describe this as pioneering, all of the groundwork for the genre was already done.

That would be like saying Beastie Boys and Eminem were rap pioneers. Or Blondie because Debbie Harry had the first chart topping single with rap music in it. First to catch the attention of white audiences? Yes. Pioneers of the genre? Not even close. We can say they were incredibly talented people who were historically significant without implying they helped create it, which they did not.