To be clear in this context, "header" is a term specific to the modding system which specifies the part of a definition that is used to identify a match from the mod files to the game files.
https://strive4power.fandom.com/wiki/Modding_Guide#Syntax_of_variables_and_their_variants:
Yes, the leading comments are limited to lines startings with a #.
https://strive4power.fandom.com/wiki/Modding_Guide#Leading_comments:
Yes, with one line of leading comment the first line is the header, and while not intended that is the only way to change or remove a header besides patching the file. Header lines were likely excluded from the count to prevent mistakes, though it does limit modding.