I guess this goes some way to answering my thought,

but it makes me wonder if there's a fundamental problem with the design of Mastodon.

I also note the article says,

During the month of November we averaged 36.86 Mbps in traffic with samples taken every hour

That seems like a large amount of bandwidth to service 30,000 users (who knows what fraction of them are actually active at any given moment). But I guess there's going to be a lot of video and image content. I have tried searching all of their linked blog posts about scaling but can't find any number that might map to requests per second without making huge assumptions.