You've got it backwards. The purpose of a UI isn't to expose internal functionality, rather the purpose of the internal functionality is to enable a user experience and user goals. Note "shutdown the computer" is not really a goal in itself, but a means to some other goal.
Regardless of whether you think purpose of the shutdown functionality is to implement the shutdown button or you think the purpose of the shutdown button is to invoke the shutdown feature, the fact is that Dr Herbie wanted to shutdown his machine and was unable to do this using the Windows8 UI.
The job of the Windows8 shell team is to translate user intentions into feature invocations. In this case they failed to do their job. This isn't a failure of the "ACPI power management team" - they still expose shutdown through the same API that they did in Windows7. The change that Dr Herbie is complaining about isn't a change of this API, or what happens behind this API, but the ability to get the shell to invoke the API through the user interface.
This is why I phrased it as the UI failed, and not the feature team failed - because the feature team didn't have anything to do with the feature's lack of discoverability. Discoverability is the sole domain of the UI (or more precisely the Shell) team.
So I stand by my assertion that in this specific instance, based on Dr Herbie's specific example where he tried and failed to get Windows8 to power down, that the Windows8 UI objectively failed compared to Windows7.