Consider one of the cites in the Wiktionary entry:
He talks about how we must remember the unsung heroes and sheroes of the Talahassee boycott, of the movement in general, and finally, he wonders how C. K. Steele would be accepted here.Suppose that the cite had simply said "unsung heroes"—what does adding "and sheroes" do for the cite? You could argue that it reminds the reader that there were both men and women involved in the boycott, and that leaving it at "unsung heroes" might not have left that impression. (As a side note, the OED does have this to say in its first definition for hero: "A man (or occas. a woman) of superhuman strength, courage, or ability, favoured by the gods; esp. one regarded as semi-divine and immortal," emphasis mine.)
Could the previous cite have read "heroes and heroines"? My sense is that in this particular context, that would have worked, at least, if the intention really was just to remind readers about both the men and women involved.
Does shero have a connotation that heroine does not? Perhaps shero is modeled on herstory, which plays on morphological coincidence (hero starts with he-, history starts with his-) to surface a term that can then be interpreted to focus on women's experiences or concerns.
I do like that theory, but I'd need quite a few more cites to try to determine whether that's actually the intended meaning.