I have to disagree, because it's not good evidence, especially if you take the entirety of the events at once.

Endsinger is a being of pure Dynamis. Driven by nothing but negative emotion. When you have your conversation with Meteion Prime just before the Final Day, she is of the exact same mindset. Then you bring in Them and make Elpis blooms, causing doubt. That causes her to falter and assist you. She doesn't influence the others successfully, and the fight continues. When Endsinger uses its ultimate, you LB3, and her response is 'Dynamis...?'

Endsinger doesn't have to be thinking you've used Dynamis. She could be asking why Dynamis is failing her. She's absorbed in her sense of right and determination, and the ultimate expression of her power doesn't work. This is a trope that shows up a lot in anime and fantasy, where the villain has a powerful doodad and the hero shows up and the it doesn't work because of some quality of the hero, and the bad guy has all these doubts in the doodad's power, questioning if it was ever really the all-powerful thing. If Endsinger thought it was your power, she could've questioned you ('How did... Dynamis...?'), but no, she just questions Dynamis itself.

The question 'Dynamis...?' by itself isn't evidence because it's unclear. A small diversion, as comparison: There's a creature that loves, say, pudding. And you end up fighting this pudding-loving creature. The only thing the creature knows about, cares about, is the drive for pudding. In the course of the fight, you get to the midpoint transition and have to LB3 to block its Needful Drool ultimate. Then this strange pudding creature says, 'Pudding...?' and the fight continues. Would you have come to the conclusion that the WoL's LB3 was made of pudding? That they had more pudding in them than this hungry creature? Of course not; this is a creature that pretty much only talks about the value of pudding, the power of pudding, the need for pudding, and is confounded by you not dying even though 'pudding in all-powerful'.

While a bit silly, it's fundamentally the conclusion you're reaching. Endsinger is about Dynamis. They talk, live, breathe, focus, and believe in the power of Dynamis. Then you don't die when it uses Dynamis to kill you, and says 'Dynamis...?' It's not saying you're using it specifically, it's saying that Dynamis failed it. And that's the crack in the armor that (with the Magic of Friendship) lets you defeat her.

I'm not saying that there couldn't be a connection, but it also cheapens Azem and Hydaelyn, where you're supposed to get the majority of the buff 'Breaking your limits as only a true Warrior of Light can' from, for the sake of a throwaway line by the end boss.

I rewatched the Elpis scene, and there's talk of despair, but nothing about your limits. I watched the one in Labrynthos as well. Can you help me find it?