Not necessarily slipping, but there is rotational flex, which will mean Z-axis errors when it raises and lowers (hysteresis errors). you can see this yourself by rotating the threaded rods by hand while the motors are on and holding position. they will rotate 5 or 10 degrees in both directions.
In an Ideal motor coupler you want some flex in position and axial angle, but ZERO rotational flex…it is hard to get that in a coupler, especially a printed one.
This coupler does look a little stiff to me. but as long as everything is aligned and your threaded rods are very straight, it should work fine.