Thanks, it mentions that it's a beta limitation so there's a possibility that when it comes out of beta it could fully work on those older devices such as iP6, IP5SE, etc.
Beta Limitations
During the iOS 12.1 beta testing period, Group FaceTime is limited to the iPhone 6s, iPhone SE, iPad mini 4, iPad Air 2, and later devices.
On the iPhone 5s, iPhone 6, iPhone 6 Plus, iPad mini 2, iPad mini 3, and iPad Air, audio is the only available option for Group FaceTime, with video unavailable.
The Camera Effects feature in FaceTime is available on iPhone 7 or later and is not available on iPad during the beta.
This how-to was last updated on October 3, 2018 to address Group FaceTime's removal from iOS 12.0 and subsequent reintroduction in the iOS 12.1 beta.