Answering “who is even using this software” is unfortunately missing in your answer. I am honestly curious. I’ve never seen it “in the wild” (in job descriptions, hearing from past colleagues, meetups etc). Only place I heard about it is HN and Twitter.
It's primarily used by people who tend to sit on the cutting edge e.g. startups and developers who follow the latest tools. It's not well worn enough to be adopted by slower enterprise environments. Bun is well known within web development but if you don't work in the space and don't keep up to date with modern tooling it's unlikely you would have awareness of it.
notably anthropic on a multibillion revenue product
I'd say the most prominent user (and the reason why Anthropic acquired Bun) is Claude Code