I tried their test site just now [0]. After clicking "I am human", my MacBook prompted me to confirm with my Apple Watch. I double-pressed the side button, but then got this error [1].
"While there is a variety of hardware security keys, our initial rollout is limited to a few devices: YubiKeys, which we had the chance to use and test; HyperFIDO keys; and Thetis FIDO U2F keys."