It allows for the editing and additional features from Unity to be used in your game using the Unity Exporter. As for the Assets you MUST still use them within the SGB Engine and NO other game engine, other than Bakin. So you could not build an entire game in Unity without using SGB, and cant say just use the assets.
You COULD however create/write an SGB add-on that uses SGB but primarily from UNITY, as long as you're STILL effectively using SGB as the base engine. I believe Alterium Shift was being made this way. The devs wrote their own editing program within Unity that allows them to edit the SGB game straight from Unity, and made it so it was easier to add their own custom code, tilesets, much easier.
I dunno what ever happened to that game, there was a couple of months of interest then the interest just kinda disappeared on the net. Then again, it might be available here on STEAM now, not looked for it in a long time.
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After all that I didnt even answer your question, oops !!
"Does this allow the use of the RPG Bakin assets in Unity by purchasing RPG Developer Bakin, Smile Game Builder and the Exporter for Unity DLC?"
You ARE allowed to use SGB assets in Bakin and vice-versa as long as you own both engines, and can edit them within Unity for the use ONLY in SGB and Bakin. You can not use altered or original assets from SGB/Bakin in ANY other engine, like Godot, Rpg Maker 3D, etc. You have to be building your game in SGB to use Unity Exporter.