A lot of its success is built on content which is not as acceptable as one might desire. See what happened to Tumblr when they cracked down.
Once you assign usernames and attach any type of value counter to an account, you've effectively dumped true anonymity out the window due to essentially creating a pseudonymic marketplace.
People may think "How pseudonymous do you need to be?" In today's world? Very. Especially given the propensity for deplatforming and reputation based social warfare.
https://www.reddit.com/user/Throwaway8675309
Also no surprise, the account was active for only ~5 days.
I think the point is that there's a distinct disconnect between your profile on Reddit (and any other sites you share the username with) and your physical person, at least up until the point where you (ideally) make the connection public yourself if desired.
It's anonymous insofar as the "person" on reddit is not easily attributable to the physical person themselves.
There is one. It was used a couple comments up.
Pseudonymous. It's an actual word, not just made up.
My point stands, though - despite the misusage of the word anonymous there's a real difference between how facebook treats identity and how reddit does. And it massively affects how communities in both interact.