Can we extract CTR parameter of an optocoupler from its SPICE model this way?
What you did is a valid way to determine the CTR for that SPICE model under specific conditions.  But there is no guarantee that the SPICE model is exactly the same as the typical value in the datasheet (which was 50%).

I have designed lots of circuits and my experiene is that SPICE models don't always caputre all the details of a real part.  Whereas the datasheet was probably based on actual testing.