I’m convinced that Master Splinter knew what he was saying to Donnie was utter bullshit, but seeing as how intelligent and temperamental Donnie was he knew he couldn’t just unleash this turtle onto the world without some sort of fail-safe.
So for the sake of humanity he would discretely try to convince Donatello that he was a peaceful turtle at heart.
Mostly so Donnie wouldn’t try to take over the world when he got older.