When did he ever say that the Republican smear machine was the sole reason Gore, Kerry, and Hillary lost?
Those attacks eroded their support and contributed to their defeat, just as they would've complicated Bernie's chances.
You purposefully misrepresented and simplified his point just so you could offer a nasty barb.
Also important to remember Gore lost by a little over 500 votes in one state. You can literally blame anything for a loss that close.
Acknowledging that the Republican smear campaign was insanely effective against those three candidates doesn't excuse them of their (or their campaign's) faults, but that's why it's dumb to assume it wouldn't happen to your favorite candidate. Like Bernie would just get on the debate stage and speak so much truthiness they would all just spontaneously combust, fucking please.
That being said I don't think there's any timeline in which Kerry would have won and that has more to do with Bush being a wartime president.
Btw I think Bernie would have beaten Trump, but that is based on nothing empirical because the last six months of 2016 would have played out completely differently. If he wanted to find out he should have won the primary.