They also says that installing a different os will invalidate the warranty. But their x86 models (I wasn’t aware of the arm) literally ship with a USB drive connected to an internal USB port which starts the setup of their custom Linux if it detects no OS on the internal drives. You just swap that pendrive and you install whatever you want. I cannot say it works for all the models, but I did a little research before buying mine and I can say it run debian for more that one year without any compatibility issue.

Terramaster is just a PC in a NAS form factor. You can install your favourite OS without any issue