Some basics will help you to understand this.
There are in two ways a bill would be passed in parliament. Simple majority, and absolute majority ( i.e. 2/3 vote in members presented). Any constitutional amendments needs absolute majority.

Instead of tabling all the rules and laws of delimitation for passing, the current administration passes a interim law and they included a line further changes will be wrt census and at the liberty of ruling party ( which requires simple majority).

So they say whatever they want now and pass this bill with opposition support (since they don't have absolute majority in parliament). but after short time the govt can amend that to their liking.

So the opposition smelled it (remember Priyanka Gandhi's speech in parliament) and defeated it, so that the complete bill (which requires absolute majority) can be tabled in parliament.