UPDATE:
Samsung recently issuied an update to DeX that came along with their regular montly one.
I ran a quick check and (as of 05/Nov/20) Zwift works in Fullscreen mode on Dex.
Also workout creator which appears to have some restrictions on screen size and is not
accessible on the phone, now can be opened when the app is in fullscreen on DeX.
Software information:
One UI version: 2.5
Android version: 10
Security patch: 1 Oct 2020
Zwift version: 1.0.57791