I'm so confused, how is this creating a pretext? A pretext is a false reason. Like if they searched for one reason but they really did it for a different one. But that's not what I'm reading here? It seems they suspected something, they used that exact reason to perform a search, and that's exactly what they found. Terrible reason, okay, but why are they being accused of lying? If a thief threatens you saying you look like you have money, and it turns out you did, and he takes it, is that a pretext too?

The pretext is that his travel to another state was assumed to be to buy weed. They appear to be relying on his existing warrant and criminal record to justify having used Flock data to not just locate him, but manufacture probable cause.

Unless they have direct evidence (photo from Flock of him ripping bong hits in his car at a light) prior to the stop to attach probable cause to, it's a pre-crime judgment.

IANAL (if you are, feel free to poke holes), but I wouldn't presume it unlikely he walks under the 4th amendment, even if it's through appeals.