I agree that it seems a bit farfetched. On the other hand, since only about 1% of people are carrying (https://www.washingtonpost.com/news/wonk/wp/2017/10/19/3-million-americans-carry-loaded-handguns-with-them-every-single-day-study-finds/) but 20% of those mass shooters stopped by potential victims are stopped by a gun, there might be some improvement that is worth exploiting. Or maybe not.

The restrictions you mention sound worth exploring independent of whether we think armed citizens are of any use it stopping mass shooters (or whether we think it's practical to remove enough guns from Texas for scarcity to be a meaningful barrier for potential mass shooters, as opposed to various sorts of background checks and cooldown periods).