I agree. I found links to a forex forum that also just screencaps / references the tweet. The closest official article I could find is her warning US debt default could see the end of dollar hegemony, but that's from months ago before the debt ceiling passed.

I could see her saying this in passing at an event, and maybe that's what people are gossiping about on Twitter. But it's not clear she said it exactly in the way the tweet presents, or where/when she said it.