Not sure the root cause but if I were to speculate it's probably because the SP4 was a pre-Threshold device and still has some legacy hooks - Microsoft is moving away from local anything - Cloud native and Cloud Services (not saying that it is the correct move)
Find my device is dependent on using a Microsoft Account (within Enterprises Azure AD Accounts are required). Is it possible that you used your Microsoft Account previously on the SP4? If so, that would explain why it shows up in the account page.
Here is the article discussing the feature as...
Have you referred to this article on MS Tech Community?
Surface Hub Windows 10 Team 2020 Update – Hub v1 status - Microsoft Tech Community
Or this Docs page?
Install Windows 10 Team 2020 Update - Surface Hub | Microsoft Docs
I had the same issue, I use my Android phone for voice and video, then add my Pro X to the call for app sharing and Whiteboard...didn't have to add any additional device into the mix...
BTW, I didn't find any ARM64 drivers for their devices