How would he know whether or not concealed carriers are armed? Is he just looking for open carriers? To this day, I believe I might have seen one person who did not appear to be LEO carrying in public.Posted? You missed the point. The shooter cased the store before hurting people. The shooter wrote in his manifesto that unarmed people are preferred to maximize casualties.
I believe nearly all of these previous high profile mass shootings have occurred in off-limit places like schools, bars or posted movie theaters. The one exception besides this one I can think off of the top of my head is the one at the church near San Antonio, and I don't know if they had posted or if they didn't post and nobody inside was carrying. The point is that there seems to be a very clear pattern of mass shooters intentionally seeking out targets where they know people will not be legally armed.
It sounds like this guy for some reason thought Wal-Mart was such a place when in point of fact it wasn't, but I guess nobody was carrying there or if they were they didn't engage him for whatever reason.