I spotted this too - disappointing when owning two of Garmins latest products (Fenix 5 Plus and Vivosmart 4) they do no play nicely together, especially as this is one area Garmin seemed to be making long overdue progress in (amalgamation and sharing of data between multiple devices).
I believe it's probably coming, just requiring a firmware upgrade, it's hard to believe that the Plus series doesn't have all of the necessary components to calculate this. "Patience, my child, patience"... :-)
