"they should limit magazines to 10 rounds"
At least he does admit:
Buried in the Federal Assault Weapons Ban's meaningless restrictions on cosmetic features was a ban on magazines of over a 10-round capacity.
The ban was porous. It did not include magazines manufactured before the ban, and such magazines were widely available.
He suggests that the AZ shooter would not have been able to aquire a 30 magazine as his was made after the ban sunset. Yet they were quote "Widely available"
In his Book: Outgunned: True Stories of Citizens Who Stood Up to Outlaws--and Won John Waters profiles the attempted bank robbery in Northfield MN, and points out due the diffculty of reloading a cap & ball revolver, that many of the men carried 6 to 8 revolvers
So even if there was a magazine limit of 10 rounds, those intent to do harm would just bring more guns to the party, or learn to reload quicker