In my experience, not all routes can be reversed using Garmin Connect Mobile, It all depends on what platform the route was created.
It shouldn't depend on the platform at all. All the platforms produce equivalent tracks and there's no magic information that one platform adds.
All of the platforms produce a track which is a list coordinates in a particular order. Each point is given a timestamp (later points have later times).
Reversing a track is simple: just write points in reverse order and update the timestamps (based on a given speed; it doesn't matter what speed).
One issue is that the reversed track might have issues with "turn guidance".
Things like oneway streets or divided highways can trip-up calculating the "turn guidance" calculation.
There was a discussion a few years ago about reversing routes and I don't recall why some routes can't be reversed in Garmin Connect. I just checked my routes in Connect and found that the only routes that can't be reversed are ones that I brought over from Ridewithgps.
From what I can tell the fēnix 6X watch just reverses the track of the course and does not take into account one-way roads and other direction related restrictions.
It would be hard to take into account one-way roads and other direction related restrictions. Just reversing the track points would generally be enough.
It is possibly a legal liability issue. Sooner or later some rider, slavishly following the directions on the device, goes the wrong way down a one-way street and ends up as road kill.
There are also those complex junctions where in one direction it is a simple left or right turn, but in the opposite direction it is a series of turns. And in the UK there are roundabouts and gyratory systems.
It's doable, but the Edge would have to recalculate the course (in effect creating a new course) and route around any directional restrictions and hazards.
