It means the brakes on the car were either of a very simple & crude design, or had parts missing. Because they consisted only of metal touching metal. No brake pads.

In this context, "with metal on metal posing as brakes" describes a car in poor condition, particularly its brakes. This phrase suggests that the brakes were so worn out that they no longer had proper brake pads, resulting in bare metal parts grinding against each other. This creates a loud, harsh sound and is very unsafe, as it severely reduces braking ability. The "posing as brakes" part adds humor, implying the brakes are failing at their job yet still pretending to function. It reinforces the idea of the car being old and unreliable, a “deathtrap.”