Key:
The Δ, being Rayquaza's Mega Evolution, was born out of "great disturbances in this world". In Japanese, Zinnia tells of the:
JAPANESE: [メガシンカのメカニズムによって引き起こされる世界の揺らぎ].
ENGLISH TRANS.: "fluctuation of the world caused by Mega Evolution mechanism"
The word used here for "fluctuation" is the the same AZ uses to describe Mega Rayquaza's origin.
JAPANESE:『世の揺らぎより生まれしもの すなわちΔ（デルタ）
人の祈りと石の絆にて世界に生まれし揺らぎを平らかにする』…と
ENGLISH TRANS.: "What was born from the fluctuations of the world, that is Delta.” “Born into the world through the prayers of people and the bonds of stones, it smooths out the fluctuations."
These fluctuations are 揺らぎ [Yuragi], which can be also translated as "tremor", in general it refers to something vibrating. The world in which Mega Evolution exists is permeated with this tremor, and the process of Mega Evolution itself smooths such fluctuations in order to give them a proper shape.
Based on what Zinnia says, we can conclude the Ultimate Weapon fired 3000 years ago was so powerful the whole world started to "tremble", almost in fear, and that's what will later allow Mega Evolution to exist.
Original can be found here: https://www.reddit.com/r/pokemonconspiracies/comments/wa6d21/commentary_understanding_the_kalos_war/
I actually disagree with the conclusion here and don't think that the firing of Ultimate Weapon being the sole cause of the ‘tremor’ is the necessary implication. If that was truly the meaning we were meant to derive it would seem to imply, when we pair Zinnia and AZ’s statements together, that Mega Evolution exists solely as both a cause and response to itself. I think this interpretation is unlikely, especially given the greater context of the meteorites already holding some latent power within them prior to being ‘activated’ by the Ultimate Weapon.
ALTERNATIVELY, and I believe more likely, the ‘tremor’ that is felt throughout this timeline is not uniquely caused by the firing of the Ultimate Weapon. Rather, it is referring to a much more nefarious, powerful, and sinister force. To me it seems Mega Evolution was introduced by some powerful force - perhaps Arceus or some other omniscient, extra-dimensional Pokémon - as a response to this unseen threat. Think of Mega Evolution as a form of power creep to help humans and Pokémon on PokéEarth deal with this force when it inevitably rears its head.
This is due to the use of Yuragi, meaning something akin to a fluctuation, tremor, continuous oscillation or vibration. In this context it seems to imply a persistent fluctuation that continues to pervade reality. This usage of Yuragi, along with the English translation’s “troubles”, indicates that AZ is referring to more than a single event. AZ states that Mega Evolution is “calming the troubles” or smoothing out these fluctuations. This wording suggests that Mega Evolution exists in opposition to a plurality or continuity of ‘fluctuation’.
In ostensible contrast, Zinnia’s initial statement about the “troubles borne of Mega Evolution” does indicate that perhaps the firing of the Ultimate Weapon is what she is specifically referring to here. It suggests that Mega Evolution itself is a distortion of the natural order. However, this does not necessarily contradict AZ’s statement about Mega Evolution “calming the troubles”. Both ideas can exist simultaneously. Mega Evolution itself can be a distortion of the natural order whilst also existing as the or part of the panacea to other greater distortions.
The firing of Ultimate Weapon is likely not the sole distortion that Mega Evolution exists as a response to. Rather, it is a necessary part of the process of Mega Evolution coming to be in the timelines where it exists. From this it should be concluded that the firing of the Ultimate Weapon is but only one in a long chain of distortions within these corrupted timelines.
When all of this is contextualised within the narrative of the Delta Episode and Zinnia’s speech about threats appearing in some worlds and not others, it can be reasonably concluded that Mega Evolution is only required to exist in some timelines and not all timelines. The same or a similar argument may be able to be made for other gimmick mechanics like Z-Moves, Dynamax and Terastallization. [more research will be done!]
NOTE: A great thanks to Kiskeym’s analysis above. Immaculate translation and research as always. I would be lost in this theory without it. Make sure to check out more of his work. It’s so good I normally never find myself disagreeing with his conclusions!