More backstory about what Pigeon Pete would call, Baxter Stockman's time as the Kraang's baker. And more clues about Stick are being revealed. Not just in this flashback but also by Yarruhk.
So how did Stockman get his hands on Splinter's DNA?
Well, just watch the "Return to New York" episode to have your answer, having been appointed Spinter's custodian by Shredder, gave Stockman ample opportunity to get the DNA.
Stockman's fly form is TOUGH to draw, but it must be the best depiction I ever did of him.