Those hypothetical numbers could be because of many reasons. One of them is that trump was the second most unpopular nominee in presidential history at that time, and faced the most unpopular. There's a reason her campaign and husband encouraged him to run in the first place.
In this case, he would have been facing someone with a +15 approval rating (as opposed to heavily negative), with no ongoing FBI investigation to drop bombshells in October, and with an opposition to free trade agreements that would have undercut one of his major areas of support - something Trump admitted on a recording years later.
Would Bernie have won by 10? No. Some attacks would have stuck, wavering Republicans would have come home. Would he have won the electoral votes by winning the three midwestern states she lost? Yes - all evidence points to it. It's better to argue with things that have evidence rather than based on vibes alone.