WTF? Samui didn't say she is immortal Madara did and now he is dead
Thats why I said you lack reading comprehension.
Madara words is not absolute. He just bragging but Kaguya was not.
Why I bring Samui here. Its because I want to tell you what is sealing.
Yes Samui is not immortal but she also not die when she get sealed inside Hagoromo's sealing tool. Samui is alive inside that sealing tool physically and spiritually.
Thats mean sealing will not kill you but trap you inside unbroken container
Many character in NV have an experience of getting sealed.
1. Kurama get sealed inside Naruto, Minato, Kushina and Mito
Physically and Spiritually. And he still alive there just couldnt move freely anywhere.
And Kurama is Juubi/Kaguya's chakra. What happen to Kaguya inside CT was her chakra got taken away. But if she still had her chakra, she can move freely inside CT like Kurama inside Naruto. And thats why Hagoromo wouldnt allowed that to happen. If Hagoromo not sealed his mother power before imprisoned her in CT, she will escape from CT.
Thats why I said Kaguya is immortal all around(Physical and Spiritual)
Remember the sealing gate inside Naruto body before Naruto released him