Which systems do you think are more similar: Windows and Linux or FreeBSD and Linux?
Presenting a Linux syscall interface on top of Windows is a hard task. The systems have fundamentally different approaches to many OS functionalities. In general you cannot just 1:1 map syscalls between the two. I am very familiar with the Windows kernel and fairly familiar with the Linux kernel and I am 100% convinced of this.
This is not to minimize the FreeBSD achievement. However I expect that mapping from Linux to FreeBSD and back would be an easier task. I should note here that I only know generalities about the FreeBSD kernel and would be willing to be educated if that is not the case and done in good faith.