I follow why someone might think that, but it turns out that's not how this works. First of all, everything deforms under every load; the only question is how much it deforms. To paraphrase Robert Hooke, all the world's a spring.
Even if you designed a car not to crumple in an impact, you wouldn't be able to sell it for street use--you'd never pass any crash tests. An infinitely stiff car would stop instantly in a collision, producing enormous G forces and killing the passengers.
As you say, the bodywork on all cars--including Tesla's future truck--absorbs impact energy by permanent crumpling or bending, AKA "plastic deformation." When an impact doesn't bend metal permanently, it still deforms, springing back into its original shape. Even this elastic deformation helps, spreading the impact energy over a longer period of time. This reduces peak Gs even if the bending isn't permanent.
But a dent-resistant body panel can actually absorb more impact energy than a standard panel. As I said before, hardness can be a tricky concept--it's probably easier to think of hardened stainless (especially the 300-series alloy in question) as strengthened stainless. Hardened/strengthened stainless takes more force to bend permanently, so it absorbs more energy than non-hardened stainless in the process of deforming.
In fact, car manufacturers use especially high-strength steels in crumple zones for exactly this reason. That's one reason the BR article ticked me off--the author casts a gimlet eye at "hardened body panels" without realizing that unibodies have been using increasingly hard/strong steel for the last 40 years.
So a thick panel made from strong steel resists dents from sledgehammers (~10 kilos hitting at ~10 m/s, or 1000 Joules) but still deforms plastically under bigger impacts (a rider/bike at 200 kilos and 60 KPH, or ~28,000 Joules). I'm glossing over some of the details here, but the Tesla truck is not going to be objectively more dangerous to cyclists than any other big truck.
As others noted, any cyclist hit by a truck is having an extraordinarily bad day, no matter who built the truck or how.
I agree with TheRich that the BR author seems to have some axe to grind with self-driving cars, Tesla in particular, Musk or all of the above. And I share TheRich's bafflement—there's plenty to criticize about all of those things without imagining nonexistent threats. If you hate Musk or fear self-driving cars, just say so.