No they don't. They could just start the debian distro without network configuration and gui and end up with a shell, with a MOTD saying this is just a base distro and a full reinstall to a supported OS is advised before connecting it to a network.
I guess they only do that because the FreeDOS name does not frighten the average tech guy who only used microsoft products as much as linux.
but the Linux they are shipping underneath that FreeDOS also works. And they also offer Linux preinstalled, so they need to "support" that to whatever amount anyways.
They probably only support the FreeDOS, not the underlying distribution. If you mess with the Linux and break the FreeDOS, that's on you. They might have 2 customers running some 40 year old legacy DOS app but 99% of people selecting the FreeDOS option are wiping it to install something else, so using it unsupported.
The pre-installed Linux works, but it doesn't have to work for all the driver and functionality edge cases that a real FreeDOS machine would definitely not support.
And the FreeDOS just works. Doesn't do much, but what it does, it does well.