He dropped Miocic with that same punch. He throws it that way to get more range, I like it a lot. It just looks like his arm is completely extended but it isnt.
It doesn't really get him more range, he could turn his knuckle over (like how Mir does in the first gif in the OP) and have a much better punch at the same distance. JDS just never worries about how his hand connects because he hits so hard it doesn't even matter. His slappy hook or overhand that connects with the thumb will still put your lights out.