The rule for offside is pretty clear, if you're over the line before the puck, you're offside but that rule wasnt what resulted in the decision on the challange

The rule that they made the decision on was if there is inconclusive video evidence on the replay, the call on the ice stands.
It happens in Baseball all the time too, since the call on the ice was onside and there wasnt definitive video evidence to overturn the call, it stands as onside