I think my argument was a bit unclear. Let me try again but feel free to skip. @oonicornflower make some solid arguments.
Here’s how I view the problem. Bullets, especially higher calibers can absolutely cause breaks and tissue damage without penetrating the body of the person being shot. This includes lethal levels of damage if the force behind the shot hits critical organs (lungs, heart, brain).
The force becomes lethal if the mass or the velocity is high enough. Baseball and hockey puck impacts to the head and chest can and have killed with surface impacts. In that case, higher mass than a bullet but lower velocity.
The bullet has a low mass but high impact force from high velocity. Rubber bullet impacts can be lethal for this same reason.
Don’t get me wrong, penetration damage is, as you point out, super lethal. You want to prevent the ammo from getting inside your squishy squishy organs and tearing things up.
And I think I was really unclear on my example with the Cybertruck. That’s on me!
I was trying to present a twofold argument that the steel plate on them can stop bullets and the impact force is not passed on between the frame and passenger. There is space and padding between the passenger and the frame, the force does not reach them.
However, body armor does not have this space. This presents the same issues (at a much smaller scale) in getting shot as a fixed steel car frame has in an vehicular accident. It’s because it has no give. No crumplezones to disperse the force behind an impact.
Modern body armor uses Kevlar and ceramic plates to stop bullets from ripping into the target and disperse the force across a wider area. The goal is to get a bruise instead of traumatic internal damage through piercing or blunt force damage.
The Toa are absolutely designed to take a hit and recover quickly. They probably do have some internal suspension for critical organs and other protections. My argument is that cannon has shown they have lungs and other squishy bits that simple plate armor could not adequately defend.
TLDR: I argue that Bulletproof in my mind accounts for preventing lethal damage from force and piercing damage. Blunt-Force Trauma Bad, Plate Armor Alone Bad For High Impact
Either way, shooting a Toa is a fucking terrible idea and they tend to travel in groups. Your villain ass is going to be handed to you.