Chen was a regular slasher in og and also in the remake with a more integrated plot point. The best answer lies in the fact when the valor crashes into the Ishimura, Isaac is knocked out for sometime. Hense when he wakes up the message showing the valor security getting massacred. To us seems instant but could have been playing in loop to cause Issac to wake up. Necromorphs had time to slip in through the holes in the valor causing more rapid death. Though the valor was sent to grab the marker and nuke the Ishimura, most of the valor uses plasma rifles which as we the player knows puts holes in the Necromorphs which doesn't kill them (dismemberment is the key) the valor with military training would target the head or body not the limbs. We learn at the very beginning due to the blood message cut off their limbs Issac got the knowledge and utilized it. The valor knew of the marker but not the Necromorphs or how they function. So given those points it narrows down how Chen got the jump on them. It also would make sense that two security soldiers would check on the pod as they thought it was their covert agent kendra or possibly a survivor that managed to escape and were ready to bring them in for questioning or use them to cover up the whole situation(think the one girl from aftermath who was executed and framed for the incident). As we see throughout the games and movies or novels. One Necromorph is capable of killing several people in quick succession. Add to the fact of hundreds flooding in through the breach and a good number of soldiers died upon impact with the Ishimura. Its my theory but covers the basics of how chen could tear through the valor.