I think this sentence is no longer valid for the release 3.5 The Administration block and Navigation block are already padlocked, as major difficulties arise if these blocks are deleted. Only unlock them if you are absolutely sure you understand the consequences and have alternative settings and navigation features in place.
The two mentioned blocks are not padlocked, but when I enter their settings I can not delete them as I can with the others (except in the Boost theme but there is the navigation drawer).
In addition, even this sentence is not very useful (but see comments about the Administration block and Navigation block in the previous section) because the link Uninstall is not available for these blocks.
Thank you :) Daniela