I think the chain of reasoning is not hard to follow:

1. Assume the Rust rewrite of Bun went badly

2. Then something must be grievously wrong with a released bun runtime based on that code

3. Claude Code uses the released bun runtime based on that code

4. From #2 and #3, something must be grievously wrong with Claude Code

5. If something were grievously wrong with Claude Code, users would reduce use of Claude Code and use alternative tools

6. From #4 and #5, users are reducing use of Claude Code and using alternative tools

7. Claude Code has wide use and use is growing across all software engineering verticals

8. #6 and #7 contradict

9. From the contradiction, the assumption in #1 is false

The parts that are not explicitly spelled out here are an exercise for the reader. It doesn't really matter if the guy who wrote Bun said this or my uncle said this.