As far I understand, by his post from gbatemp, the sole developer wants to get the project to certain point, and then expand it and bring new peope in. At that point going open source could be a good idea, as it would make developement with a larger and shifting team easier.
If they still want to keep it private, I would venture, that when the original developer no longer wants to work on it (for private reasons or simply motivation), they would likely open it up. Has been the case with a few emulators until know. (Even Dolphin, if I'm not mistaken)
So "likely never" seems unlikely. The open source model has too many advantages long term. (Though short term, especially when the code base is still small, closed source might have its advantages too)