Shanks and Mihawk were rivals. They were.
This is when Shanks was a measly pirate with 1 billion bounties, there is nothing in the manga that suggests that they are currently rivals. The manga makes it clear that they competed in the past. Until Shanks started using Roger's fighting style, from then on he never bothered cutting things again, when he has a power more overwhelming than that.
Oda never said anything about Shanks' sword skills, stop lying
1 Billion is anything but measly. Shanks at that period of time already was a powerful pirate. And the fact someone like Mihawk, who holds great honor in swordsmanship, had Shanks as a rival, is testament to Shanks' swordskills. The first databook (or was it the second?) talked about Shanks being a top class swordsman, with the manga referring to Shanks' swordskill when introducing Mihawk's bounty.
The reason they are not rivals anymore, is due to Shanks sacrificing his arm for another cause. Now I wonder, why would Shanks during their rivalry adhere to swordsmanship (I believe their duels were described as them honing their skills ("sharpening their blades") and afterwards suddenly change it up and use Roger's moves? Why wouldn't he use Roger's moves during the battles with Mihawk? So it's either Shanks said during their rivalry, that he will restrict himself from using shockwaves, but that would be weird, or he randomly picked up those techniques later after his rivalry, but why would he? He already knew of those abilities since his childhood, being on Roger's ship.