The way I read it, they left the other functionality unchanged and injected their source code change into the .dll source, made the build process build both the hacked version and the unhacked version (for later) and put the hacked version into the installation package and deleted evidence of the hacked dll in the build and restored the unhacked dll to it's place so no one could tell what happened just by looking at the end results and doing a dump or binary diff on just that dll.
Similar to but not the same as the pirated xcode malware injection.