Why does anyone think it's likelier that Sanders people will split the Democracts than Anti-Trump people will split the Republicans?
Both will happen, to an extent. Ideologically, many of the sanderistas are a lot closer to Trump than they are to any conventional Democrat, including Hillary Clinton (or, for that matter, Barack Obama).
Maybe they are ideologically closer to Trump if you weigh ideology like you do; that doesn't mean they'd vote for Trump. There is just as much an identity politics factor that would keep young white BernieBros away from Trump as would keep ideologically close blacks from voting for him too. Peer pressure and group identification will pull them grumbling into line just as easily as Pubs were pulled to Trump.