I think that, given the law as it is written in TX, the 911 call from an "alarmed citizen" was probable cause for the stop. It was at the officers discretion to enforce (8) displays a firearm or other deadly weapon in a public place in a manner calculated to alarm.
I think you are misreading/misapplying the law. Read it again - "IN A MANNER CALCULATED TO ALARM" is the critical element of the crime. What evidence is there that Sgt. Grisham was carrying his rifle in such a manner? Are you suggesting that the mere use of a single point sling translates to such intent?
Btw..the 911 caller(s)' "fear" is irrelevant. It matters not how afraid they are - the only issue is whether Sgt. Grisham's open carry of a long arm was intended to cause fear. Absent evidence to the contrary, one can only conclude there was no reasonable suspicion for the arresting officers to infer such a method/manner of carry, and therefore they had no legal basis on which to conduct a Terry stop.