This happens when our gradients are no longer gradients. This is what I mean.
Look at this setup
As you can see, these nodes are creating a square. Now, what the color dodge node is doing is that it is controlling the fall-off of the gradient or you can say the bevel of your displacement. Look what happens to the displacement when I change its value:
Now, if I put the value to 1, the square becomes a solid color. That is when displacement gets messed up.
What you should do with the displacement of this texture is that use the displace modifier for accurate geometry. Not all textures should be used with the displacement node only. There is another way.
In this instance, take a plane. Then make sure you have adaptive subdivision on. Then, go to the modifiers and add a "displace" modifier. What that will do is that it will allow you to displace your texture as real geometry. This is what I mean. Once you add the modifier, this is how it is going to look like. Your plane is going to change its location: (btw, im using the 1k version of the rocks due to my potato computer, hehe)
Now, in the displace modifier, click on the "new" button.
Then, click on this button:
It should then open the texture tab. Now click here:
Then browse the displacement texture of the rock texture you downloaded.
You should now see this:
Now, go back to the modifier tab and lower the strength of the displace modifier.
You should now get something that looks more better.
However, you still see the lines, even if a little (use auto smooth to get even less lines). And I think that is because there is the "gradient" problem (I mentioned above) in the rock displacement texture.By bluring the texture, you can get rid of that. Open any photo editing software and blue the texture. And then repeat the above-mentioned steps with this new blurred texture.
This is what you should have once you use the blurred image (keep in mind that you need to blur it "very" slightly):
As you can see the lines are totally gone
Here is the final result: