1. We learned that you can guard with supreme kings haki, and the only way to surpass that guard is to hit it with even stronger supreme king haki.
2. Shanks told whitebeard he was on guard and not being careless.
At this point u tell me since I'm the one full of copium. How else do u see a pre df bb permanently scaring shanks who was on guard
Now add to that this chapter.
3. Shanks might have imu tax and before u say thats just the first level he only had teleportation my first two points still stand. In order to fight against and do damage to someone like shanks ur haki has to surpass his.