(Edited: mixed it up on the last sentence.)
Which is exactly what the post says this is. It's running Windows 9x on Linux kernel. It's strangely worded, but from the follow up comment, and the readme in the repo it seems clear that it's running on the Linux kernel.
Pretty sure it's the other way around. But I haven't had my coffee yet ;)
Microsoft's naming scheme confuses me every single time though: "Windows Subsystem for Linux" actually runs Linux on Windows...
I missed the part that it runs on the bloody Windows 9x kernel, I was to busy thinking about modern Windows.
Per wongarsu's post, something like the OS/2 Subsystem is an OS/2 system with Windows beneath it, but the OS/2 Subsystem is much smaller and less consequential, thus subsidiary (in the auxiliary sense) to Windows as a whole.
Isn't marketing fun?
This is how we end up with hundreds of products that provide "solutions" to your business problems and "retain customers" and upwards of a dozen other similar phrases they all slather on their frontpages, even though one is a distributed database, one is a metrics analysis system, one handles usage-based billing, one is a consulting service, one is a hosted provider for authentication... so frustrating trying to figure out just what a product is sometimes with naming conventions that make "Windows Subsystem for Linux" look like a paragon of clarity. At least "Linux" was directly referenced and it wasn't Windows Subsystem for Alternate Binary Formats or something.