set_disable_timing usage
First, the difference between set_false_path and set_disable_timing commands:
set_false_path command will remove all timing constraints from the path. Tools still calculate the path delay, but don't report it to be an error. While set_disable_timing will remove paths from timing analysis. So you can see set_disable_timing is more efficient than set_false_path command.
Second, in the example circuit, U1/A is a reset signal, which has no timing requirement and has a large delay to U1/Z. The transition time of U2 is affected by the cell delay of U1, which is calculated on the largest timing path, that is U1/A to U1/Z. So if not disable it, the cell delay of U2 will be much bigger than actual. The false error may hide the real violation.