Yep that cable is the speedo one & will have nothing to doe with the clutch operation
Right Ho .... Here's a brief insight into how the cluch works
You have a friction plate that sits against the flywheel the central part is fitted via the input shaft of the gearbox.
This is held in place by a sprung loaded clutch cover .... Thus trapping the friction plate against the flywheel & providing drive from the engine to the gearbox.
You have said when you press the pedal, the release bearing makes contact with with the clutch covers spring fingers ..... This is normal .... The lever now needs to move a little further forward & will diss-engage the friction plate.
When the fingers on the clutch cover are depressed it releases the pressure on the friction plate, this in turn is free to spin & will stop the engine driving the gearbox .
I don't expect you have ..... Make sure you have put the friction plate the right way against the flywheel
The flat side of the friction plate must face the flywheel
If the release bearing is free to move on the input shaft, it may be a case of exerting a bit more pressure on the pedal to see what happens !!