By that definition, almost every user on every social media platform would be an "employee". What reddit mods are doing is really not that different from creating a facebook group to organise a party with your friend or a twitter page to share curated content that you love.
Mods don't work for reddit, they work for their community/subreddit. Creating a group on a social media platform doesn't make you an employee that can be fired. It makes you a power user that can get banned if you mess with other users/communities or with the platform too much.
I know one person personally who is a mod. It sure does look like work. He is effectively on-call and always tending to fires. I can imagine this to be a minority but yeah, I thought of it as "his job"
Mods don't work for reddit, they work for their community/subreddit. Creating a group on a social media platform doesn't make you an employee that can be fired. It makes you a power user that can get banned if you mess with other users/communities or with the platform too much.