Cutting an AMOLED screen in a circle isn't exactly rocket engineering. Driving it will be a bit of a challenge, but not something that a team of electrical engineers can't overcome. The biggest problem is that it's going to be expensive, which is convenient because a smartwatch is a luxury item and people will be willing to pay if it's even halfway decent.
The biggest problem is that they haven't demonstrated it will be even halfway decent yet, and none of the existing comparables give any confidence to that.
How can it seem to be anything, it's not even real yet. It's a marketing mockup.
AMOLED fits the bill easily because the technology allows you to place dots however you need to place dots and doesn't restrict you to arrays or rectangular shaped emitters or even planar geometry. The only challenge is running wires to each LED to drive it.
And that's not the screen. Your linked screen is 240^2 pixels, while Moto circular display is is 320^2 pixels. Additionally, that screen has too big a bezel (especially on the bottom) to fit inside the moto's case.
The biggest problem is that they haven't demonstrated it will be even halfway decent yet, and none of the existing comparables give any confidence to that.