Pointless article. Sanders isn't the race to win and he knows this, otherwise he would have run as an Independent. He wants to debate Clinton on national TV.
His chances are obviously extremely slim regardless, but he'd have a better shot at winning as a Democrat than an independent. If he somehow managed to win the nomination, he'd at least start with 40-45% of the vote. As an independent he'd be lucky to crack 5%.
I'd say his slim chances are higher as independent. The intrigue of a serious independent candidacy brings out high turnout from people who normally check out of the political process. 1992 was a huge year.
He'd have an easier time convincing 30% of Democrats, 30% of Independents, and 10% of Republicans in a general than 51% of Democrats to roll the dice on him being able to win. Both impossible scenarios, but one slightly less so.
The Republican candidate wins in that case, since they keep 90% of the Republican vote and most of the remaining 70% of independents.