To begin with, Vivzie didn't openly state that the Hierarchy was about power; at most, she compared it to a food chain, but the concept of power in the Hierarchy is something that is poorly assumed and indicated by the series.
Interestingly, one example of this is precisely the fight between the Imp and Andrealphus. Andrealphus's dragon is clearly superior to the Imp (and they would have died if it weren't for Octavia), even with access to Goetic weapons. Furthermore, he only took damage from Loona in her demonic form with piercing AP from her bite and from Octavia's magic (although he partially tanked it). Both Octavia and the Imp made it very clear that it was shameful for a Goetia like him to have trouble with Imps and a Hellhound, which he agrees with.
It doesn't mean it's exact; it can be exceeded and has its exceptions, but it's a good average.
But beyond the Hierarchy, we also have the mention of magic being possessed by stronger Demons, which is also corroborated by the series. Whether it's Roland being stronger than Millie and Blitzø, who are able to kill common Hellborns with relative ease, or the Overlords having magic as a clear characteristic in Hear My Hope.
Considering these two quotes, plus Charlie being the daughter of the God Tier of the verse and second in the Hierarchy (a victim of Alastor), it makes sense that she would at least surpass an Moxxie or Millie or Blitzø in base form.