They vibecoded it, and admitted as much. Once it was able to self-vibecode, that's all they did. That's why it's written in React and uses gigabytes of RAM as a chat client.
You say that as if somehow the trend for cross platform desktop apps to be ridiculously bloated bundles of browser overlays is new?
What major cross platform app isn't based on Electron or Tauri? Slack, Discord, VS Code, Teams, Notion...
Not only did they decide to write a terminal application in React, but it's 500K lines of code. It's strange because I'm sure Claude is capable of writing a decent TUI in C. It says a lot about the engineering culture at Anthropic, at least on the software side.