There are some people who want the Democratic Party to be more progressive. This is generally a good thing. These are the people who, for example, rather quickly ensured that the DNC chieftain race quickly coalesced around two staunchly progressive candidates in Tom Perez and Keith Ellison.
There are some people who want the Democratic Party to simply lash out in a blind fury against "the establishment". These are the people who, for example, went far beyond having a preference between Perez and Ellison and crossed into a groundless and seemingly random fervor insisting that Tom Perez was unacceptably right-wing because something-something-most-progressive-Labor-Secretary-in-recent-memory-is-neoliberal, and swore to dynamite the entire Democratic Party if he won (by the way, Democrats just picked up their 37th special election seat-flip since Trump was elected after swinging a Kentucky state House seat 86 points from its 2016 presidential margin).
The people who endorse Dennis Kucinich -- Dennis Kucinich!, shilling for Assad and Putin when he isn't playing the "Trump is speaking to real American outrage" card* -- over Richard Cordray for the Ohio Democratic gubernatorial nomination are definitively in the latter category.
As always, the question isn't whether Cordray is perfect (though one expects the Jill Stein bait-and-switch -- laboring to make a mountain out of a molehill's worth of liberal heresy when the candidate is "establishment" while resolutely ignoring all the ways the "insurgent" is ideologically terrible in her own right -- is coming). But there's very little reason why a Elizabeth Warren-esque former head of the Consumer Financial Protection Bureau should be so unacceptable as to provoke a turn to a cartoon character like Kucinich (in this way, it's far worse than Perez/Ellison, as at least there the alternative candidate was perfectly fine in his own right). The motive rather seems to be just an undirected form of contrariness towards anyone who establishment Democrats are content with -- if they like him, then he must be unacceptable for ... reasons (probably something to do with neoliberalism). But that's not actually a way of building a progressive movement.
Loomis is too generous in saying it's a problem of progressives not being "smart" (though it is profoundly stupid). Independent of it being bad tactics, it's also bad on the level of ideals. It's difficult to know what to do with that sort of blind self-destructiveness (again, shades of the Stein voters who answered the question "why should I vote for the lesser of two evils" by electing to vote for the middle of three). The best thing that can be said about it is that so far, it hasn't actually had that much influence on actual Democratic voting patterns (which is more than can be said about the Republican Party, which has been entirely consumed by a pure id of reactionary anti-"establishment" ressentiment).
* Researching this post, I rediscovered that Kucinich was one of 12 Democrats to vote against the House resolution of "disapproval" after Rep. Joe Wilson (R-SC) famously exclaimed "you lie!" in the middle of President Obama's State of the Union. So add that to the list.