Saw this today, working with making WinPE v"whatever" using the Windows 10 ADK. If running on a Windows 7 SP1 x64 host, DISM will skip driver signature enforcement when servicing a newer WinPE equivalent image.
This is helpful and not at the same time. I think for a WinPE, it isn't a big deal but it would be for an OS. Using DISM from Win8/8.1 ADK, you'd get a certificate error when trying to inject a driver signed by the WDK for 8/8.1 if the host PC was Win7 or Server 2012 SP2. It wasn't an error per-se, but basically the host was not able to interpret the certificate on a digitally signed driver and thus threw the error. If you did the injection on a Win8/2012 R2 host, there would be no problem.
I am just doing testing now, so I hope that this doesn't happen on a 2012 host because I don't really want to build a VM with Win10 or 8.1 just to inject digitally signed drivers into OS images. Note: the reason this is an issue is certain channels are unable to disable signature enforcement on OS images.