"Summoning those Abyssal little sisters, is not as interesting as summoning Deities."
The Abyssal little sisters mentioned by the Dragon Cub, he knew them, Little Vampire Tiji, Little Demon Princess Bettina, Little Fallen Angel Mevis, Little Medusa Freya.
These little princesses now possess decent strength, capable of beating up the young dragons from Dragon Island. If they were not demons, it wouldn't matter for the Dragon Cub to summon them.
The problem is that they are demons. If the Dragon Cub summons them to face the young dragons on Dragon Island, that changes everything.
Dragon Island would think the Dragon Cub is the vanguard of an Abyssal Demon invasion of Dragon Island, and might even suspect that the Dragon Cub has been brainwashed by an Abyssal Demon or that its body has been stolen.
In this case, Dragon Island might let dragons aged one thousand or two thousand hit the young dragon to death.