It seems that this is directional, flowing from Android to Apple but not necessarily back (e.g., me airdropping a photo to my parent who uses Android). I'd love for this to work in the other direction as well.

There's a gif on the blog showing file sharing in both directions. Apparently "Contacts only" sharing doesn't work yet, as mentioned in another comment: https://news.ycombinator.com/item?id=45995586

The video shows both directions.

I came to the same conclusion when I clicked the link to try it out, just watching the video now to verify that the flow is both ways.

The demo shows it working both ways, so you're in luck