Do you know where the term "bulletproof" comes from?
Given your comment, I doubt it. So allow me to reduce your ignorance on the subject.
Back in the days when late plate armor and early guns overlapped many knights worried that their armor would be useless against a gun, that the bullet would simply pass right through their armor and kill them. It was a legitimate fear. To reassure those knights, armorers who were selling breast plates would actually fire a gun at said breast plate, leaving a mark/dent/whatever you wanna call it behind on the armor. This was the "proof" mark that the armor would stop bullets.
Now, of course, historically the improvement of firearms eventually rendered plate maile obsolete. So I can understand your frustration. But, I don't understand your lack of imagination. Sure, what that person is wearing _looks_ like old school plate armor. But looks are often decieving. For all we know there isn't even a guy under that armor. Maybe it's animated. Maybe it's magic. Maybe it's some fancy pants ceramic and polymer blend that is effective at stopping bullets with just that mass. Maybe it's for a place in time where technology is so advanced it lets you make your armor and weapon LOOK like anything you want it to, but function like tanking power armor and a 100 kilo joule rail gun.
In short there are all kinds of reasons to combine plate armor and guns, from the historically accurate to the science fictiony to the utterly fantastical. Your consternation at such a combination is mystifying.