How to implement the manual step at the end of continuous delivery?
Personally I consider the distribution of the software to a target just an intermediary step of a deployment - installation/activation of that software being necessary to complete that deployment.

To me the delivery (as in continuos delivery) stops when the software to be deployed is created and made available for deployment (i.e. for distribution, installation and activation)

So, to answer your 1st question, no, I wouldn't consider distribution and installation as reflecting the manual step differentiating continous delivery from continous deployment.

Yes, in some (hopefully rare) cases that manual step is just the final human decision for deploying into production, reflecting the cultural mistrust in process automation and the mental comfort of having a human double-checking and signing off the deployment decision (thus assuming responsability for it) even if that decision is purely made based on an algorithm that can be automated (like counting pass/fail testing results).

But in general it simply reflects the fact that the decision for performing the deployment in production is not simply the result of an automated algorithm. Here would be some example such cases:


the automated decision is overwritten 


deployment can be signed off even if not all quality criteria are met (we all know that this is not just a theoretical case)
the deployment is held for whatever reason, even if all criteria is met (due to market timing implications for example)

the automated algorithm is not (yet) implemented/deployed
the algorithm includes checks for some criteria depending on human decisions (say results from manual testing)
deployment is actually done in a 3rd party customer environment, following additional customer checks


So I wouldn't look at the manual step simply as an implementation issue.