To me it is by definition a custom driver, from the link you posted:
WinBtrfs is a Windows driver for the next-generation Linux filesystem Btrfs. A reimplementation from scratch, it contains no code from the Linux kernel
Also you can note I didn’t recommend using ntfs-3g implementation I specifically talked about NTFS3, which indeed may have bugs, but I disagree on your position.
I wouldn’t work on Linux file systems from Windows, and I would trust more the NTFS3 driver by a corporation that made working on file systems for almost 30 years its core business, than a driver rewritten from scratch on a GitHub project by one man. At the end of the day you do what you want but I don’t see how that makes more sense “the other way around”.