It's semantics, but the word "baritone" generally means either "the second lowest pitched instrument in a family of instruments", or "an instrument pitched between bass and tenor". So technically if you tune a bass 6 to the same low E as a bass, it's a bass guitar, not a baritone guitar.
But in practice, it's strung with strings with lower tension, similar to a guitar, and is often played more like a guitar, so it is what it is.