Now that the Primordial Nightmare Contract was established, Jacob can sense Primordial Nightmare's emotions just like the Brain Hunter, which also gave him some confidence about Primordial Nightmare. But he still didn't know its abilities and what else it could do, or how he would be able to hatch it.
The Beast of Primordial Nightmare was still an egg; it could only rely on its intentions through this strange mental ability. Its body still hadn't developed in all these years, which was quite strange. Yet despite all that, it has awakened its intelligence and could use abilities like Primordial Nightmare Contract.
This alone would prove how talented and terrifying the Primordial Nightmare is, and letting his guard down would be a mistake unless he confirmed the authenticity of the Primordial Nightmare Contract like the Eternal Enslavement of Brain Hunter.
He even wondered if every terrifying being has such a weakness to ensure they never get out of control.