The question is regression to what. When it is taught that bodhisattvas on the 8th bhumi don't fall back, it means leaving behind the possibility of becoming an arhat. Non-regression on the 1st bhumi means guaranteed liberation, either as a sravaka or as a buddha. Not falling back from the initial awakening of bodhicitta means maintaining the determination to enlightenment and not leaving behind the path of liberation completely by rejecting the Three Jewels. This is probably different from how Kongtrul understood the issue.