They aren't right.
Hilary was a vulnerable, unloved candidate who had multiple never ending "scandals", was in the middle of an FBI investigation with a last minute blow from James Comey, interference from foreign governments, decades of smears, and made serious campaign mistakes but still won the popular vote. Trump is less popular today than he was in 2016, losing a lot of ground in rural middle-American especially, with states like Wisconsin and Ohio polling like they will flip. Florida actually has a million+ new potential voters, mostly minority and left leaning, re-enfranchised.
Any candidate the Dems will put up of the major players here should walk Donald Trump, but if it would definitely help if the Bernie bros in particular don't get as shitty about another candidate as they did about HRC.