The position of his hand in that photo and him being a Fascists seem more like coincidence rather than being related.
There are non-NAZI, valid in that situation, arm and hand movements which yield still photos that look the same as a Heil Hitler, for example waving for people to "cool down" (arm to the front and slightly to the side moving up and down, hand open, palm down).
(This for example does not apply to the actual video of Musk's NAZI salute, since a NAZI salute is the only arm and hand movement that matches what was shown in that video of Musk - in the face of that whole video the universe of alternative explanations is zero)
So this still of Hegseth with his hand in that position does not logically prove the guy is a NAZI or even a Fascist because there are other entirelly valid non-NAZI hand movements that would yield such a still.
Now, if that was a video of the entire thing and it was the same whole movement as Musk did, that's a whole different story.
It's what Hegseth says (including in this specific instance) and his actions that prove that he's at the very least a Fascist, not this single shot photograph.