Bidirectional is a lot like biweekly. Biweekly depending on context means twice a week or once every two weeks and bidirectional can both mean per direction and total of both directions.

But yes I meant 512Gbps each way, to be clear.

I'm only a single datapoint but I've never encountered that usage. My understanding of a bidirectional link is that it meets the same spec in both directions simultaneously. It's important precisely because many links aren't bidirectional, sharing a single physical link between two logical links.