This line is the problem:
# See sudoers(5) for more information on "#include" directives:#includedir /etc/sudoers.d
The #includedir statement needs to be on it’s own line:
#includedir /etc/sudoers.d
It’s a “special” comment that actually means read all files under /etc/sudoers.d. Without it the /etc/sudoers.d/jupyterhub-admins file is ignored.