Cell isn't Majin Buu; he doesn't absorb people for any reason other than to become complete or suck out their life essence. You need only to look at the reactions of everybody and the statements from Android 16 to see that Cell was considered the strongest after Kamiccolo.If Cell was stronger he would have just absorbed him. Vegeta was likely his equal or stronger. I don't know where it was 'stated' Cell was stronger.