I suspect in the end, since they're not advertising this "feature", the Cybertruck is likely more-resistant-than-average to bullets, but nothing that could be legally advertised.
edit: Not a rumor after all, Musk specifically mentions in this clip: https://youtu.be/25ZuKkbHdqM?t=167
When metal is used, it is typically in a thick plate. Even thinner plate used for small round target practice is usually painted- stainless would be an odd choice even at the right thickness.
That's not the same thing as specifically choosing stainless steel to make it bullet proof. He's using bullet proof to help sell the stainless steel, not the other way around.
It's a tiny distinction, but an important one IMHO.
A 45 is a low-velocity round with a very large blunt nose.
If they shot it up with an ak47 round or a 5.56 out of a AR15, they would have made Swiss cheese out of that door panel.
There are many high-velocity pistol rounds that would do this also. One is a 5.7mm or a .22 magnum."
from that same article
They have talked about it on the keynote.