About it being an outlier, there are a few points.
Guy, one of the two people she is fighting at the time, is said by rimuru to be able to maybe fight and win against yuuki, who is significantly stronger than all of them, to the point that yuuki would have been easily capable of eating it, which was a universal busting attack, which is high 3-A/low 2-C
Not only that, even chloe gets stronger, incomparable to her past self in which she was in a complete different dimension of power (not saying tier 1)
Although maybe exaggerated as the next person she fought she couldnt beat, she still amped insanely higher
Rimuru also says velda is the strongest person he knew at the time:
In which she was capable of fighting him 1 v 1 for a period of time.
Also milim's dragon nova attack which is large star level is more often done with her only utilizing a portion of her energy; in which velgrynd says it is about 10% of a true dragons, and if she used her ultimate skill, she'd surpass them:
(Literally says her power would multiply, and in the next scan says it would beyond them, and 10×10 to reach the same level as a true dragon would still mean 10× stronger than a normal large star as she was still capable of producing bigger ones as her final plan before using her ultimate ability included gathering energy)
In which she couldnt imagine how high her strength would rise.
1. This literally already says guy compares to yuuki which is a low 2-C feat.
2. Milim's large star feat is significantly weaker compared to her ultimate ability enhanced AP which chloe scales to as she fought 1 on 1 with velda
3. Enhanced true dragons also said that milim buff was beyond what she could imagine
4. She compared to guy before milim went into her ultimate form which automatically makes her higher than high 4-C
5. Stated to be incomparable when unsealing her manas
I know it must be a bother but I have to disagree with it being an outlier, even veldora had low 2-C feats later, thats yuuki low 2-C, guy low 2-C according to rimuru.
As for the atmospheric prison, it still shook the "world" and yes your interpretation says the planet, but often in later chapters, it actually refers to the planets using "planets" not the world, even describing evil dragon lorf aji dahaka said it almost reached the level to eat planets:
Then later even says 4 people could create an explosion comparable to the event horizon/big bang:
That was diablo, dino, zegion and benimaru. Diablo being the strongest still isnt as strong as guy. And for my interpretation, I think the world does stand for the universe, no contradictions of it suspended world not effecting the entire universe as it affects universal laws, previous context implying it was the universe, raw kanji being looked over and only having one line to which they agree it likely means the universe.