Does hypoxia by itself lead to cardiac arrest?

Potentially. Hypoxia means an inadequate amount of oxygen in the blood. Is can lead to vital organs not receiving enough oxygen to survive. If the heart is starved for oxygen this can lead to ischemia or heart cell death which can cause either failure or an abnormal rhythm which leads to a cardiac arrest.
If severe enough. If the oxygen level is low enough, it can cause the heart to stop. But, it has to be quite low for that to occur.
Possibly... The heart is a muscle which does need an oxygen supply for it to function correctly. This oxygen is delivered to the heart by the coronary arteries. So, if there is less oxygen delivered to the heart muscle, it could go into an abnormal rhythmn which in turn could lead to cardiac arrest (meaning the heart stops beating).