Well, I don't know about a document on that particular image, but those OEM images are just ESXi with specific drivers bundled.
The good news is you can inspect the IBM image yourself using the Image Builder CLI. You can even customize it to your liking, if you don't like what they've included/excluded.
Using the vSphere ESXi Image Builder CLI | VMware vSphere Blog - VMware Blogs
One option would be to grab a vanilla image and the IBM image, and use Compare-EsxImageProfile to spit out the differences.