They write directly to relays, but they read the notes over a caching server. Which is like a relay, but I think not really because it enriches the notes, rather than just storing them for a while and passing them on when you request. I don't understand why they don't just use an aggregating relay, like theforest.

I'm glad I took time to understand relays better, because that doesn't make sense to me either. I wish I could give more input on that🤷. What do you mean by enrich the notes?