Is nobody gonna talk about the fact that the loli devil instantly exposed the secret that the hero has kept secret for five years (or however old the reincarnated hero is now)?
To put it simply, the devils are very individualistic and they're not friend of the demons.
Cannibal's pact is just between him and the first Demon King (and inheritor of the soul eater spear).
Other devils don't give a damn about him and his contract, that's his own business.
And he never promise anything about protecting the demon race.
What they do care is the resulting dark portal means demons looking for power now can come to the devils' world.
If the visitor's soul is strong enough, and the devil likes their personality, they will make a contract. The devils get benefit because the contract, and more direct action towards the material world helps them grow strong faster.
So, Ante got no reason to snitch on Alex.
Odigos (mister fabulous stick) also seems to understand Alex' identity and motivation, but he only care about showing the way to anyone in need.