You got it right. As you say, they had hands on the net while ball was going thru. that's not any different than having a hand pulling on the rim as the ball goes down thru. The key here is that the outside influence (hand pulling on the net) does not directly cause the BI, but its where the ball is relative to that action. If the action caused interferes with the ball in any way, then its BI. In picturing what you described, think what would have been said if no call was made...
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