For reference my work laptop (8 logical cores, so 4+HT? 32GB RAM) can handle 100k rows/second sustained inserts into postgres 14 with some batch jobs I'm working on. You can buffer http requests into batches and easily handle way more than 10k/s on a server while still providing synchronous semantics and reasonable latency to the client (e.g. flush batches every 10-100 ms).

I doubt Mastodon is designed for that kind of scalability, but most techies could probably afford to run Twitter as a hobby if they knew what they're doing and they weren't trying to do all the analytics and advertising stuff to monetize it/just wanted to provide the service.