There’s no “entry-meta” style used in your theme as a container for author information and categories/tags, which is kind of a standard for WordPress themes, so that these entities can be selectively styled/hidden via CSS. Also, there’s not a single entity that includes “By authorname” without also including the categories, so even if you program CSS specifically for Graphene, there’s no way to hide one without the other.
Also, you have a “li” element with class “byline”, but it contains both the byline and categories. This is generated in inc/loop.php, and uses the values of get_the_author without testing whether they returned false — which would call for not displaying author information.
This means plugins such as Hide/Remove Metadata, which attempt to suppress the byline by hooking into get_the_author, the_author, etcetera, again can’t suppress just the author name — they always get rid of the category also.
And this code is in an enormous source file. I certainly don’t want to create a subtheme that replaces that entire file. I’m kind of new at this, so it’s unclear whether I can replace just one function that contains your erroneous code. But I feel I shouldn’t have to. If I want to turn off the byline on a page by page basis, it should be possible to do it using a plugin or some page-specific CSS, and it just isn’t.