There are exceptetions to that law: https://en.m.wikipedia.org/wiki/Strafgesetzbuch_section_86a

Subsection (1) shall not be applicable if the means of propaganda or the act serves to further civil enlightenment, to avert unconstitutional aims, to promote art or science, research or teaching, reporting about current historical events or similar purposes

If there are reasons for German newspapers to omit Musk’s Hitler salute, that’s not it.