I found this[1] repo which tries to reproduce it, and its accompanying blog post[2] uses (z_t, z_t - delta), without further mention.
Another repo[3] trying to reproduce it also does the reverse from what I can see, so (current, past).
Interestingly the first repo got underwhelming results, while the second repo matched the paper roughly. Not sure if it's related to the Claude-usage or not in the first, though it notes a lot of details were left out of the paper which is true.
[1]: https://github.com/Infatoshi/grassmann-flows/
[2]: https://github.com/Infatoshi/grassmann-flows/blob/main/blog....