*sigh*
Let's go back to the basics, since you need to have your hand held:
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So, after Kaguya attained the power of the holy fruit - she gave it up, yielding it to her sons.
Since Naruto and Sasuke are still alive - it means 'her power is not maximum.' Which was reinforced by the statements in the current chapter.
So, obviously - she was pretty ****ing powerful even before consuming the chakra fruit.
Since we're on the topic of Kaguya and fruit:
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What, pray tell, could she require an army for? These are zetsu we are talking about - compared to Kaguya.
If there was no chakra amongst humans to begin with... then ... what good would IT do, and how did anyone know that it was necessary to produce a new fruit (and where did the first fruit come from - what processes created it)?
And... if IT was used before... who stopped it? How?
Why would Kaguya share the chakra fruit between her two sons - then go ape shit trying to get it back? Why the change of heart?
Why does Black Zetsu - a being created when Hagoromo sealed Kaguya (and who was suspiciously aware of and not surprised by the fact that Hagoromo was sight-seeing in the future) - seem to be calling the shots for said goddess obsessed with power and control?
To understand what is going on - you have to look at what inspired the author to include the symbols he does, their meaning, and the connection they have with the story.
For example - the square in Japanese lore is the symbol for the waking state and for the sun goddess, Amaterasu.
The square is the symbol used for the Hyuuga seal over the third eye. The third eye - notated by a circle - is the symbol for the human mind and faculty - ruled by the moon god, Tsukuyomi.
The interaction of sun and moon create the shifting tides - the tempest - Susano'o, Lord of the Stars, notated by the triangle. The stars are navigational aids - reminding one of the need to be awake - to be present.
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It actually took me a long while to decipher the key.
At first - it was obviously an arrangement of the bagua (eight trigrams) about the symbol for yin and yang (indicated by the spiral). Bluntly a Hyuuga reference. It is worth noting that according to the Later Heaven arrangement of the bagua - the 'missing' element (the one with all the text/symbols drawn out in its place) is that of thunder/lightning - the element of division - hence the purpose of the key to alter the division of Naruto and the Kyuubi's chakra.
It is a strong hint that the Uzumaki and the Hyuuga were working together, at one point - since this is an Uzumaki Seal with many Hyuuga/Taoist overtones.
But the 'key to the key' is the square.
Amaterasu. The sun - shines upon the moon - yin - which produces the phases of the moon, Tsukuyomi (the spiral). This creates the Four Movements that interact to create the Eight Trigrams - which cycle amongst themselves to create the mundane world (the triangles - susano'o).
This is why Susano'o blocks Infinite Tsukuyomi - the stars are the guide back to the waking present.
The Hyuuga seal was created to block the development of the third eye - of Tsukuyomi. Of course - it could also be argued that it is a symbol for simply being 'awake' - but it would seem unlikely.
The key implies that whoever created it (the Uzumaki - and seemingly the Hyuuga) had an advanced understanding of the physical naruto-verse and likely the mechanics of Tsukuyomi, as well (or at least, they had enough knowledge to guess very accurately).
"But what does that have to do with the Byakugan?"
Well - if you would find it within yourself to desire to learn something, for once - you could set about researching Taoism. Since, you know, that's where the whole "Eight Trigrams" thing comes from.
Look at the philosophy it teaches, at the legends involved, and at how the mechanics explained in Taoism rest at the very core of the Naruto universe. Even the gods work according to Taoism.
Then we can begin to apply simple "anime logic."
The things which are least explained yet so fundamental tend to come to a head very late in the story. The challenges and the characters involved tend to get progressively more powerful as their abilities jump orders of magnitude with each new challenge.
The rinnegan has been explained.
The sharingan has been explained.
The Senju have been explained.
As have the Uchiha.
The Uzumaki remain ambiguous.
The Hyuuga remain ambiguous.
The Byakugan has yet to be explained.
Uzumaki sealing techniques have yet to be explained.
Obviously, the 'big baddie' is going to involve a Byakugan and therefor be obscenely powerful because the abilities relate to the manipulation of the core world of the series. Which is precisely what we're seeing.
But, it's okay. Kishimoto just lost his mind and doesn't know what he's doing, making random shit up. I understand.