As per my own search and understanding, the issue is not purely related to the file format (fit or gpx) but more related to the fact that Suunto routes seems not to manage coordinates along with distance or timestamps (any of them would suffice) (as @sky-runner is explaining) (gpx could do this as well as @nigel-taylor-0 is saying also).
Fit or gpx, it is more how routes and Waypoints are handled in Suunto routing current mecanisms. Currently, if I did understand well, WP are some kind POI on the route (track) but without “when will they appear on the route” data.
But I can be corrected, it is not recent since I have looked into this.