I checked the D-Bot build guide and F623ZZ flanged bearings are the recommended idlers and I'm sure that's what I have. They have a diameter of 10mm which explains the artifacts. I'll pass it on so hopefully the Bill of Materials can be updated. Thanks Bot!

I'm not at home at the moment but they're small – similar in size to the diameter of my 16T pulleys. A larger idler might help. Here is some additional context from a response I posted on Reddit:

"Keep in mind that this is a black, glossy filament that was printed too hot (making it less viscous / more susceptible to deflection) and the light has to be oriented just right to catch the surface ripples.

It is worst when rotating prints by 45 degrees so each stepper motor is isolated and printing at higher speeds. In a typical orientation (flat surfaces parallel to the X and Y axis), the effect is more subtle and may just appear as slight surface texture (I suspect this occurs if the motors are offset by half a step). On cylindrical surfaces it looks more like moire or diagonally arcing defects.

I didn't notice the issue with PLA printed at the low temperatures and speeds or using lighter colors. It can also be alleviated by loosening the belts at the expense of jerk / acceleration artifacts with higher speeds.
It only became a problem as I tightened the belts down while testing the upper end of the speed range for the D-Bot (150 - 200 mm/s). Then it drove me nuts..."

After an extended struggle, I just cracked an issue that looks like this on my D-Bot (CoreXY). The issue was GT2 teeth hitting smooth idlers and the solution was a half twist in the belt so the smooth surface would contact the idlers instead of the teeth. Note: the pitch of the defect also didn't align perfectly with the GT2 teeth so I think the interaction of the teeth on the idler is more complex than a simple 1:1 with the surface defects. The album has all of the details including the (many) things that were ruled out before arriving at a solution: https://imgur.com/a/kb2nE

Here's the side by side shot showing the result of the belt twist:

I also posted this to Reddit. If you're interested the discussion links are:
https://www.reddit.com/r/dbotcorexy/comments/7d1s8j/vertical_surface_ripples_banding_waves_solved/
https://www.reddit.com/r/3Dprinting/comments/7d1pqe/vertical_surface_ripples_banding_waves_solved/

I hope it helps!