Incident Command Ops are built to transfer IC to more qualified leadership as it come on scene. Arrendondo could have tried to do that, but there might have not been a clear chain of authority here. I would have thought DPS or Texas Ranger at the very least. I doubt seriously he was IC through the whole incident. If he was, then that's a state failure to not have that handover training in place. I know our local school goes through this training every year and firm plans are in place as to first response through to the final ops. I'm not defending Arrendondo, but I can also see how this could break down easily with muddied communication and unsure of who has authority to issue commands.
The problem with ICS is that it is cumbersome and intricate, and it's easy to forget where your spot is in the system as new personnel are added. Most of mine is based on firefighting and hazardous material incident responses.
ICS is a great tool, but it's also something that takes a lot of time to put into place, and it is not a good situation for OJT or on the job refreshers. It has to be practiced often and in more than just one scenario. Not only gunmen, but dirty bombs, WMD, biologicals, etc. All those are threats.