As you said in your post, he was legal. Therefore there existed no suspicion of illegality, and therefore there was no reasonable suspicion to detain.
If the police account is true, he wasn't detained until he refused to be disarmed during the course of investigating a complaint by another citizen. If correct, then his refusal resulted in him being detained. The videos provided by Grisham do not contradict the police claims nor does to video support his claims that the police tried to disarm him by force with no verbal request. The video shows none of this because it happened BEFORE the camera was used.