Yeah it is credible. There is no indication that he clearly got away or thought he clearly got away. The fact is smugglers aren't very bright. He most likely didn't return fire because Ramos hit him first and didn't want get hit again so he continued to run into Mexico.
But Ramos wasn't the only, OR the first, to shoot at him. Compean shot first, 14 times, and never hit him. If getting HIT is what stopped him from firing, he would have fired at Compean. IF getting shot at made him give up and run again, he wouldn't have turned around AGAIN to "point at" Ramos.
And if he wasn't going to shoot, why would he pull the gun at all? Never point a gun if you aren't ready to use it.
You don't have to be bright to know that if you point a handgun at an officer, they aren't going to put down their weapons and run away, they are going to SHOOT you.
And everybody knows that if a drug smuggler points a gun, they are going to shoot. After all, if perps regularly pointed guns at cops without shooting, then the BP agents would have no justification for shooting the guy for simply pointing a weapon at him.