The function is_author() is used to check if you are on the archive page for the specified author. So as your code is in single.php this will never be true.
From the docs
This Conditional Tag checks if an Author archive page is being
displayed. This is a boolean function, meaning it returns either TRUE
or FALSE.
https://codex.wordpress.org/Function_Reference/is_author
To get the author ID you could use:
$author_id = get_post_field('post_author', get_the_ID());
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