In the Gospel of Judas, Judas isn’t simply the 13th Demon, his star is attached to the 13th realm which means that the 13th realm in the Gospel of Judas is a cosmic one, not one of the pleromic realms above the cosmos. Stars are fixtures of this universe and govern it. There are no stars in the pleroma. Furthermore, the Gospel of Judas says that Nebro(el)-Ialdabaoth and Saklas lived in clouds and with their six assistants generated twelve angels in the heavens and gave them to the twelve angels for dwelling places. This means that Nebro(el)-Ialdabaoth and Saklas live above the 12th heaven in the 13th realm of fixed stars that is referred as the 13th realm in which Judas' star dwells. This isn't the cosmology just in this text, but is also found in other Sethian texts as I explain in my book.
The reference to thirteen “seals” in Marsanes, is not the same reference. We shouldn't be mixing them up. The seals do not represent 13 cosmic realms, but refer to the fact that the initiate must be sealed 13 times in a ritual activity. Furthermore in this late text, the first three seals are associated with the cosmos and ascending beyond it. The fourth and fifth seals concern the place of disembodied souls and repentant souls. The sixth seal is the seal that belongs to the self-generated aeons. And so forth.