A SYN flood attack inundates a site with SYN segments containing forged (spoofed) IP source addresses with nonexistent or unreachable addresses. (Juniper Reference)
Another important bit of information, the SRX is not a proxy server per say (hence no feature to enable), it will either proxy and queue or drop SYN segments request but only when the SYN Flood protection kicks in, hence the requirement for SYN Flood protection. Each SYN segment request will result in more sessions in the session table.
Example of Basic Process: The SRX will NAT traffic to a host on the private network and pass on the initial SYN segment to the internal host, the host SYN/ACK segment response will be returned to the internet based client and the SRX will keep a session open waiting for the ACK to return from this internet based client which it returns to the internal host when received. The SRX does not actually respond with SYN/ACK segments during this process itself it only forwards on the requests between client and host.
In a FLOOD The SRX will either start proxying the SYNC/ACK response in an SRX based queue or it will drop the SYN segment packets depending on the configuration of source/destination and ingress port. Proxying will only happen once the SYN Flood protection kicks in until then the SRX is vulnerable to SYN Floods.
If you don't configure SYN Flood protection there is no proxying of packets in a queue or dropping excess packets.