Using just a browser is way too token intensive and slow. It would look for 401 errors then run the browser automation to login with the credentials and grab the token.
Did you clone the Reddit API from browser traffic and then turn it into a 100% API driven thing?
I'd imagine they'd be sniffing browser agents, plugins, cookies, etc. to fingerprint. Using JavaScript scroll position, browsing rate and patterns, etc.
Maybe their protections just aren't that sophisticated.
The application-layer stuff is harder. Each application can develop its own heuristics, and that's difficult to automate in a cross-cutting fashion.
Reddit doesn't do anything about that? That seems stupid.