tbf it uses the zebedee library that is mostly fiatjaf but it looks like they wrote their own relay core
this is the relay implementation:
https://github.com/HORNET-Storage/hornet-storage
this is the codebase that implements this frangible variant of merkle trees:
https://github.com/HORNET-Storage/scionic-merkletree
i don't know exactly how well it works, haven't tested it, and actually i'm kinda facepalming right now because i was actually almost working with him but he never offered to pay me, i did some stuff and had a share of a bounty dangled at me and all this led to bad blood between me and him
but now i'm looking at it... here's a Go relay codebase that IS NOT from fiatjaf or mattn...
gonna give it a good poke
Discussion
also, i had a bit of a look at it... not really that fussed over their architecture or naming schemes, and it looks like they use libp2p and ipfs as part of the protocol... didn't really want to dig too deep into it.. and it is worth emphasising that it's still early beta, but their GUI for managing the relay is pretty nice from what i saw
also, it has early stages of also implementing negentropy in it too