Just how it works isn’t immediately apparent. A video included with the feature suggests that both the iris scanner and camera are active at the same time regardless of the conditions, but it’s not certain whether this means combining their data or using one as a backup for the other. The iris scanner might take precedence at night, for instance.
As the feature isn’t currently exposed to users, it might be introduced with the S9 when it’s launched in February. That doesn’t rule out support on older phones, to be clear — Samsung could give the feature to S8 and Note 8 users whenever their official Oreo upgrade arrives. The question is whether or not they’ll get the same level of face recognition-based features. The Exynos 9810 chip headed to the S9 (and possibly the Snapdragon 845) enables iPhone X-style 3D face detection and a secure biometric processing unit, so you might need an S9 to use your face for payments or other features that depend on stricter security.