I've found the relevant
affidavit now. It certainly makes interesting reading, and suggests that while the cabin crew did not complete a full headcount (!) US customs and border control did after the fact and accounted for all other passengers, meaning he must have avoided gate security:
The CBP control booth was responsible for keeping track
of all the passengers arriving into the United States from every
international flight, and where the passengers were in the
processing system within CBP (...) CBP officers in the control
booth queried all of the passengers from SK 931, along with all
the passengers who had arrived at LAX on every European flight
before 3:00 p.m. on November 4, 2023. The control booth
confirmed that all of those passengers were accounted for.
OCHIGAVA had not yet processed through CBP at the time that CBP
showed that 100% of the passengers who arrived from Europe
before 3:00 p.m. had been accounted for. According to CBP
records, there were no additional passengers from SK 931 to
process, and no additional passengers to process who arrived
from Europe before 3:00 p.m.