It was most likely NOT a protective vest, but a simple tactical vest with pockets for magazines.
“Military-style” means it looked scary.
What makes a weapon deadly is its operator. My bolt action scout would have been even more effective in the hands of a competent operator—IER Scope, effective cartridge, fast reloading.
Oh, almost forgot—the operator can hold 2 MOA with it out to any likely range in an urban environment.
Apparently the guy used a vintage SKS with a 10 round mag that is even legal in CA.