I think they agree with you about what “imposter syndrome” means, but think the shift from “imposter phenomenon” to “imposter syndrome” turned it from something that would be understood as imposed, to something that would be understood as personal. I have no idea about the history of the term, or whether their description is true or not. Just, I’m pretty sure that is what they are saying.

It's minutia they dug up that almost no one knows or cares about and they're using it to put their own spin on it for the sake of whatever argument they're making.

Ask them to explain how they expect syndrome to have displaced phenomenon when "imposter phenomenon" is practically non-existent in common usage and knowledge. If we can judge by the trend alone it would seem the reverse may become true.

https://trends.google.com/explore?q=imposter%2520syndrome%2C...