https://soranews24.com/2019/07/19/kyoto-animation-security-system-was-shut-down-on-day-of-arson-attack-that-killed-33/?fbclid=IwAR2UXzlUf2shgYNe1wAPXVP7fRvYPLcR7cb6lkyfUo8bCKYE3lPjGCSJEbc
Their security system happened to be shut down on the day of the attack. They did have one in place, like ID entry, but that day they were expecting news coverage to come and the system was intentionally shut down to eliminate hassle. There was only one fire exit on the first floor where roughly 1/10 of the people were occupying at the time, and the arsonist set fire to the stairway. That, on top of the roof door being permanently locked, transformed the building into an oven.
So yeah, he brought people unrelated to KyoAni into this! There's no certainty as to whether or not he knew they would shut off the security system days prior or if he was just lucky.