I think they went after Hunter on "gun charges" because then it looks like they are prosecuting both sides equally, but they can let Hunter off with a slap on the wrist because the left doesn't want a Biden or a Democrat punished and the right doesn't want someone punished for an error on a government form especially if that form is related to gun control.
I think there is a long list of crimes for which Hunter should be punished, but inaccurately filling out a government form that I don't think should exist is not one of those crimes.