Since syslog-ng apparently forks over to the supervising process the correct type is forking as the service's exec start is expected to exit immediately after execution with the child process running the service.
Looking at the status for the service show both the supervising and actual process.
● syslog-ng.service - System Logger Daemon
Loaded: loaded (/etc/systemd/system/syslog-ng.service; enabled; vendor preset: enabled)
Active: active (running) since Thu 2019-05-16 19:26:08 UTC; 14s ago
Docs: man:syslog-ng(8)
Process: 12591 ExecStart=/opt/syslog-ng/sbin/syslog-ng $SYSLOGNG_OPTIONS --cfgfile $SYSLOGNG_CONF_FILE --control $SYSLOGNG_CONTROL_FILE --persist-file $SYSLOGNG_PERSIST_FILE --pidfile $SYSLOGNG_PID_FILE (code=exited, status=0/SUCCESS)
Main PID: 12592 (syslog-ng)
CGroup: /system.slice/syslog-ng.service
├─12592 supervising syslog-ng
└─12593 /opt/syslog-ng/sbin/syslog-ng --no-caps --cfgfile /opt/syslog-ng/etc/syslog-ng.conf --control /opt/syslog-ng/var//syslog-ng.ctl --persist-file /opt/syslog-ng/var//syslog-ng.persist --pidfile /var/run/...
May 16 19:26:08 ip-172-30-6-133.ec2.internal systemd[1]: Starting System Logger Daemon...
May 16 19:26:08 ip-172-30-6-133.ec2.internal systemd[1]: Started System Logger Daemon.