She endorsed Bernie, and criticized the DNC for putting their thumb on the scale in the primary
Yeah, this is literally all there is to it. In Hardcore Bernie World, if you're a politician who supported Bernie - regardless of your personal or political positions and whether any are progressive or not - you're a True Progressive. If you're somebody with an identical record or have a comparable number of ideological transgressions as "Generic Progressive" but who didn't support Bernie, you're a neoliberal hack.
It's really not complicated. It's also why a) the push for ideological purity from these supporters is so laughable and b) why this movement is very likely to continue falling flat on its face in terms of enacting legislation/policy. They'll just keep supporting anybody who kisses the ring as "progressive" (i.e. supporting Bernie and/or a handful of other figures) and then get all mad when half the candidates that they actually do succeed at putting into office don't turn out to support them on a particular initiative.
Well, as I said upthread, I don’t think that captures the full picture. Sure, some just blindly back Gabbard because she endorsed Sanders. But there’s definitely a strain of Gabbard fandom which has joined the Gabbard cult for reasons that have little to do with Sanders. Read the commentary at RunTulsiRun, and other FB and Reddit groups, you’ll see the folks who have a Glenn Greenwald-esque view of the world, where they are anti-establishmentarian above all else, and their principle attraction to Gabbard that elevates her above other progressives is her rejection of the bipartisan foreign policy consensus.
I mean, they like her on other things too, like health reform and the Dakota Access Pipeline, etc., but it’s her foreign policy views that set her more apart from most of the rest of Congress. Some of these folks will even talk about how Sanders is too wishy washy on foreign policy.
EDIT: E.g., here is someone who's deep in conspiracy-land and strongly backs Gabbard:
https://medium.com/@marshallbart/tulsi-gabbard-is-americas-last-best-hope-deb62b45efb6His column actually ends with "We are living in The Matrix and Tulsi is The One."