I was under the impression that Horizon created the soundtrack and, as you say, it was not advertised that they took it from elsewhere.
This is a great example of my biggest issue with HorizonXI and its team. Anything that you've enjoyed about that server is something that they had
nothing to do with, and any changes or original updates for the server itself (outside of its launcher, to their credit) is poop.
The music remixes of the zones that already had music is all great, but that was all done over a decade ago by someone completely unaffiliated with the HorizonXI team. The decision to use that orchestral listening music to replace background music, the decision to add music to zones that didn't need music, and the crappy original songs for those zones
are the credit of the HorizonXI team.
Just like how the game assets are great, but those were made by the original FFXI team. The server database functions, but that was made by the LSB and ASB teams. You can't praise Aerec and Friends for any of that. But you know what you can credit them for? The awful job balance changes, the ugly new character models, the awful economy changes, the weird bugs, and so on.