At first, it's helpful to put musicians into stylistic boxes:
"Diane Krall is a jazz pianist."
"John Legend plays contemporary pop music."
"Bernie Worrell was a funk pioneer."
"Johnnie Johnson played rock and roll."
This is an entirely valid approach to understanding the musical world. There are so many pianists and keyboard players out there that we need a way to organize it all in our minds, both stylistically and historically. Jelly Roll Morton and Keith Emerson, for example, lived many decades apart and were from different geographical regions and musical cultures.
But after a while, this approach begins to limit our understanding.
Diana Krall also sings, is married to rock legend Elvis Costello, and has recorded an album of pop originals herself.
John Legend is an all-around musician who has played jazz, gospel, and R&B in addition to the pop songs he's known for.
Parliament/Funkadelic's Bernie Worrell was a classical prodigy as a child and also loved to play jazz.
Johnnie Johnson was a jazz and blues pianist who incorporated these styles into his playing as a member of Chuck Berry's early rock and roll group.
Musicians don't have the same musical boundaries that listeners often have. Sure, musical labels, boxes, and boundaries can be a useful tool while we're beginning to learn about all these great musicians and the music they make. Just be sure to relax these labels at some point, and enjoy their wonderful music on its own terms, wherever they themselves went, musically. Then, you can do the same!