There's no sight of Aeternus and the legions of the house of Khaos no matter how she and everyone looked. The disappearance of Aeternus is one thing. It might be that he somehow made himself invisible but an entire army of millions of demons can't just up and disappear.
The hilarious possibility that Aeternus bypassed the divine plane didn't come across the demon lord's mind because it is ridiculous and hilarious. Anyone that tries to make her believe that a channel large enough for Aeternus and his army was opened for him will fail in the endeavor to convince her. She is not a fool so how can she believe such a ridiculous tale?
She won't even believe it if someone tells her that a channel was opened for Aeternus alone. A channel for a single demon may have a higher chance of succeeding but it is still far-fetched because it will mean Aeternus had connections within this plane. It is highly unlikely considering that he has never left the 541st plane of the abyss.
Those connections also have to be very high too. Aeternus must know a Celestial to achieve that but what will a Celestial be doing with him a high-rank demon when there's a demon Lord in the plane and several demon kings in the abyss? Even then, his entering into the plane must not be opposed by other Celestials. In fact, the only way it is certainly feasible for a channel to be opened for Aeternus is that the majority of the Celestials are for giving him access directly or indirectly. Any Celestial can open a channel and any Celestial can close it. Its success becomes a matter of majority support. So no, she will not believe that Aeternus bypassed the divine plane.
The only possibility, even though it is extremely difficult is that Aeternus managed to fool her about his destination. It is a more likely explanation despite the fact that it pains her to admit it. It is disappointing and shameful to admit that Aeternus was able to pull a fast one on her despite her carefulness and the anticipation that it could happen. But it is more plausible than anything else and she is not so prideful as to blind herself to the truth of the situation.
So she informed the demon nobles, "It is a bust. The lord of the house of Khaos isn't here. It seems that I underestimated him once again and he fooled me. We should try and return to the abyss when we can."
Their operation is officially a failure. Their prey escaped from them and it is not a pleasant experience especially since they have committed a lot of resources to this. But they have to leave as quickly as possible or they might be forced to stay here forever.
Beelta began the return protocol even before she told the demon nobles to retreat. She cares about herself more than she cares about others. It is a good decision too because the angels are already attacking.
It doesn't matter to the gods that the demon lord made a mistake. All that matters is that she is an invader and must be vanquished. She might try her luck by explaining to the gods that they didn't invade to attack the plane. That is unlikely to work since demons are known to be liars and gods generally hate them.
No one will believe her if she tries to explain. The demons rampaging in the mortal realm have sealed the god's mind to any other alternative explanation and their Celestial Supreme is just looking forward to getting a bite out of them. No matter how good Beelta is as a liar, no one will believe an explanation that is as implausible as invading a plane with billions of demons and doing it by mistake.
Beelta didn't try to pally. She knows that this fight is inevitable. She ordered her soldiers to engage the defenders while she stayed at the back of the army in peace. So millions of angels clashed with millions of demons and the battle of Armageddon began. It looks like an epic battle for the fate of the plane except that the demons are being sacrificed to buy time. All the leaders of the demon army are trying to return to the abyss. In about a day when the leaders start disappearing, this farce will be undone and revealed for what it truly is.
CELESTIAL SUPREME POV
The battle in the divine plane is of little significance to the Celestial Supreme. It is something that he can win easily but he held back his most powerful troops. The Archangels were stopped from participating in the battle for fear of demon trickery.
Common sense dictates that a demon king is needed to beat the gods but there is no demon king in sight. This demon lord is not a threat so maybe all of this is a plan to make him overconfident of victory and make a mistake. He doesn't want to send out the Archangels into the midst of enemies only for demon kings to appear and corner them.
The fight might take longer but the remaining angelic troops are more than enough for the invaders to be destroyed. Besides, battles like this are not fought within a day and these demons aren't going anywhere. He is more concerned about what the contingent of demons that entered the plane will be doing especially since the mortal realm is vulnerable right now.
He was right to be mindful. The demons that entered the mortal realm show just how conniving and treacherous demons are. An army that is supposed to have only high-rank demons as the strongest combatants turned out to be false. The divine plane failed to sense demon nobles and demon dukes in that army.
Somehow, the demons fooled the detection mechanism of the divine plane and were able to smuggle in powerful demons. It is like the sensitivity of detection mechanism of the plane was scrambled by a weird energy used in opening the portal. It is the first clue that the Celestial Supreme got that there is something peculiar about the demon invasion.