It was an interesting fact of the 2016 primary that Bernie Sanders performed best among two sorts of voters: those who thought Barack Obama was too conservative, and those who thought he was too liberal. The first makes intuitive sense -- Sanders was running to the left, and so it stands to reason that he would be appealing to voters who wished Obama was more left than he was. The second seems strange and contradictory to the first (but it explains Sanders' big wins in states like West Virginia). What gives?
One way of telling the story is that Sanders' narrative of unapologetic economic populism and uncompromising attacks on establishment power brokers -- including those in the Democratic Party itself -- speak a language that can turn (White) working class voters into progressives. This is an appealing narrative particularly for Sanders' more Marxist-oriented backers, for whom it remains a nettlesome embarrassment that the White working class doesn't seem that interested in backing progressive candidates. The idea that the problem was just that "we haven't tried it yet" obviously has its allure.
Another way of telling the story, however, is that Sanders' approach attracts conservative White men not because it compels them to abandon their conservatism, but rather qua their conservatism. The anti-establishment tenor simply meshes well with their conservative priors, which includes a deep belief that "the system" is out to get them and is stacked against them. It is of a kind with, not disassociated from, racial resentment and misogynist backlash -- they are very willing to believe that women and minorities are part of the big bad power structure that's intruding on their turf and responsible for their ongoing misery.
Part of this boils down to what I think is an intuitive-but-mistaken understanding of how political coalitions form. If we think of voters on a continuum from most liberal to most conservative, then political campaigns have two plausible routes to increasing their vote share: they can seek to pick off the marginal voter (i.e., Democrats should target the most liberal voters who voted Republican in the last cycle -- which is to say, centrist swing voters) or they can boost turnout among their partisans (i.e., the "mobilize women of color" approach). It would be weird to think of building a coalition of "liberals and a chunk of reactionary right-wingers", skipping over moderates entirely. But in reality, most citizens' political identities are fragmented, and so there is plenty of room for "reactionary right-wingers" to back unconventional progressives (or vice versa) if they organize their campaigns along the right axis -- anti-establishment populism being a very good candidate for that axis. "Horseshoe theory" is a version of this, though I think it oversimplifies -- but it is no accident the continued affiliations we see between, e.g., Melenchon and Le Pen backers, most recently in the "Yellow Vest" protests. Melenchon and Le Pen backers see something in common with each other and each other's politics (see also: Corbyn's obvious preference for Brexit). That means that either one probably could do better at attracting the voters of the other than their more "moderate" peers -- but when they do so, it isn't because Le Pen voters are really closet socialists if only given the chance to express it.
The thing is, I think Sanders backers are right to both think it is plausible that Sanders can uniquely appeal to a class of voters that Democrats have long written off as unwinnable, and that his ability to do that can rightly be viewed as a major asset in a game where the goal is to win as many votes as possible (the question is whether his gains amongst that cadre will be offset by losses among "centrists", but I think it is plausible that he'll come out ahead). But I also think they are too optimistic in thinking that the reason he'd win those votes is because these voters are attracted to a "progressive" vision as they probably understand it. In reality, a Sanders-led progressive coalition would almost certainly include a significant reactionary strain -- a contingent of voters for whom Sanders is attractive because the "establishment" they take him to be tackling is one that includes uppity women and overbearing people of color and snotty intellectuals and, yes, liberal academics.
Democracy, one might say, is about uncomfortable coalitions sometimes, and certainly the "standard" Democratic coalition also entails its share of unsavory compromises. So I don't think this should be viewed as some unforgivable flaw in his electoral organizing. But I do think it's important to be clear-eyed about it.