Not true, 2 sword style ha Rushomon which is basically upgraded version of Shi Shi Son Son
Plus more swords Zoro has, more competent he is, he has literally commented on that again and again. Even all the way back in EB when Sham and Bucchi were overpowering one sword Zoro but 3 sword Zoro neg diffed them.
Btw to show you how much of a gap between one sword Zoro and G2 Luffy is, check this out
Not to mention one sword Zoro one shotting that Dragon on whom Haki G2 attacks were useless on
Oh and one sword Zoro did better against Fuji than Luffy did with G3.
One sword Zoro overall is G2/G3 Level while Santoryu Zoro is G4 boundman level.