He was, in fact, arrested for asking to see a warrant, that is clearly documented.
The claims of assault that DHS fabricated and published on social media and via other channels after the fact to justify it, of which there is no evidence, before Lander was released without any charges are interesting in terms of understanding the current regime's propaganda propensity, but have nothing to do with explaining the events clearly captured on video.
I don't think that's what they were referring to. From watching the video, I assume it was when he grabbed onto the fellow they were detaining and refused to let go.
If you witness police misconduct, you do not have the right to impede the police. Regardless, there was no way for him to form an accurate belief about whether misconduct was occurring. The agents have no duty to provide a warrant to a bystander, even if he is a government official.
The claims of assault that DHS fabricated and published on social media and via other channels after the fact to justify it, of which there is no evidence, before Lander was released without any charges are interesting in terms of understanding the current regime's propaganda propensity, but have nothing to do with explaining the events clearly captured on video.