Well, in this particular case everyone working with the new tileset would be required to sign a multilateral NDA which is basically them saying they won't disclose anything even something as little as a username.

A multilateral DNA is basically when one party (tileset creator/Unixmad) knows they'll be disclosing information to at least 2 other parties (multiple people on the staff team) and want to protect the information from further disclosure (staff team to players).

In order to make this work the NDA will have applicable punishment for anyone who fails to respect the agreement in the case of Graal it would most likely be something simple like having the right to ban a staff member from all Eurocenter/Toonslab services.

They can also terminate any staff members who don't agree to sign it.

Obviously they didn't do this because it's just too much work to be physically getting signatures from everyone, but this is a common practice in game development.

They probably just won't let anyone see the tileset.