Wielding a pipe isn’t a fighting style in of itself, not even for Sabo, he uses that in conjunction with his martial arts. The latter is actually what is fighting style is, the pipe is just a complementary aspect. Same goes for swordsmen. They have a fighting style of their own where swordsmanship dictates how they fight, regardless of what they use in conjunction to complement their swordsmanship. Shanks was never once seen fighting without his sword, thus he qualifies as one and the “duels” he had with Mihawk for all intents and purposes confirm his initial pursuit of the title.