I worked at VMware for over 11 years and was one of the first dozen or so people on the vmkernel team (which produced ESX and later ESXi), although I left several years ago.

If I recall correctly, the source tarball was created from a simple script which pulled in a bunch of open source files which were stuck into their own separate branch in the source repository.

To my knowledge, there wasn't any linux source in the vmkernel. We did use plenty of open source code, such as busybox, but my understanding was it was always in user space and the source was always published.

It's possible that Christoph's code was just pulled in and published with a bunch of other crap that wasn't actually in the product, but someone had checked it into the open source part of the source tree.