Now that I think about it, the biology behind pipis is not that hard to understand. Since they come from Spamton only, it's safe to assume he's the only one who can lay/create them. If they're not fertilized, they only have Spamton's dna in them, so it makes them hatch quicker (that's why the pipis you destroy in battle hatch into little Spamtons).
Also the little angel Spamtons come from unfertilized pipis, and the wings are a genetic mutation.
IF the pipis Tenna has saved and takes care of is fertilized with both of their dna, it's safe to assume that it can hatch into an actual new darkner. A perfect mix of Tenna's and Spamton's dna baby.