I'm not familiar with Texas, nor am I claiming it's relevant to why he was arrested. However irrelevant to to the prosecution isn't necessarily irrelevant to the defense.
But it's unnecessary in the context of this case. The sergeant doesn't have to justify his possession of a rifle, and it cannot be used as some sort of precursor to an officer's claim of reasonable suspicion in the lawfulness of the arrest. We have the arresting officer on video confirming to the sergeant that he is being arrested for lawful conduct.
If that isn't enough to satisfy folks' as to the illegality of the police officers' conduct, then this is an exercise in futility.