As I've said before I'm not an nginx expert... but my reading of the docs is that that syntax is also only supported since 1.11.0.
Well, strictly speaking it's worse, as 256 is smaller than 384, but one might hope that the difference is mostly irrelevant in practice. Ideally you would upgrade to an nginx version that allows you to support multiple curves, so you can use the best one supported by the client. Or even better, the developers of ARC should improve their app to support more and better curves.