Currently, Community SmartApps (including ActionTiles) can access Scenes only indirectly by
(a) first assigning the Scene to a Routine. It can be a trivial Routine that dies nothing else but Activate Scene. Thus you’d need SmartApp user to create a new redundant Routine for every Scene
OR
(b) Using Smart Lighting to trigger the Scene based on a real or Virtual Switch or other other Trigger (eg Location Mode) that SmartApps official can call via command.
You are correct in noting that Smart Lighting has direct access to activate Scenes.
Besides this, I suggested it would be handy if Scenes automatically appeared to be Routines just to SmartApps; that way all existing SmartApps which have select and execute Routine in their code… would be immediately compatible with Scenes - no code changes required, and no manual effort required to create wrapper-Routines.
I hope that my suggestion is under consideration.