Metion and her sisters were created by Hermes, so therefore ancients can interact with dynamis in that way, they can make creations with thin aether and dense dynamis.
Elpis flowers (made of dynamis) reacted to the ancients, they are always white in Elpis unless Hermes is near and they pick up on his depression. That’s a major plot point and proves that ancients can influence dynamis creations.
In Ultima Thule, a zone that we are led to believe is purely dynamis, Emet & Hythlo show up to create Elpis flowers using creation magic AND 'weave the fabric of reality' to make the zone survivable for us (oxygen etc.) as seen in this scene:
Hythlodeus:
“As you’ve called us to the stage, so shall we perform. And creation magicks never fail to please.”
“Drawing upon the hopes of your comrades, we will make for you a new path.”
Emet-Selch:
“What form that path takes depends on you. So focus - focus and envision that which rejects the claim that you cannot atain your goal.”
Hythlodeus:
“Ours is the wisdom to weave the fabric of reality…”
“Ours is the power to create!”
Emet *SNAP* The field of elpis flowers appears. (Dynamis flowers, created in a dynamis zone by aether dense ancients.)
Emet-Selch (later in this scene):
“In case the practical implications are lost on you, your comrades no longer need to fight their fight.”
“So go on, call them back to your side.”
So their power is not limited simple creation magic even whilst inside a dynamis influenced zone, they are easily able to alter that zone itself into one that is fully survivable for aether based creatures.
Also, judging by the fact that Zodiark's Aether shield held up for 12,000 years against the endsingers onslaught, despite Zodiark being sundered, seems to show that Aether is strong against dynamis and in no way shows that as a weakness.
So, where does the story prove the theory that "the ancients couldn't defeat a being made of nearly pure Dynamis"? I guess with my bad reading comprehension or short attention span I must have missed it. ¯\_(ツ)_/¯