That's the thing is, whether the God Joyboy actually existed or not is not so important.
Devil fruit abilities are born of mortal desires for something, now factor in the desire of potentially hundreds of thousands, maybe even millions of slaves desire for a warrior to set them free. All that energy directed into a single being gave rise to the Nika Nika no mi
Perhaps then Mythical Devil fruits are born when there is a mass desire for something, or mass belief in something. Devil fruits are born of the power of imagination, and the nika fruit is the embodiment of imagination.
Honestly this is a much better justification for an ability to be overpowered