In current CMake releases:
After some error checking add_dependencies results in a call to Target->AddUtility(). x is added to the list of utilities for my-lib.
target_link_libraries does not result in a call to AddUtility, but it does add the arguments to the LINK_LIBRARIES target property.
Later, both the content of the LINK_LIBRARIES target property and the list of utilities are used to compute the dependencies of the target in cmComputeTargetDepends.
The list of utilities in a target can not be queried at configure time, and is only used at generate time, so the use of add_dependencies with arguments which are libraries already added with target_link_libraries is redundant.