https://cs.opensource.google/go/go/+/refs/tags/go1.24.3:src/...
The answer is that errors.New just wraps the error message in an errorString struct, and the second line of `is` is a string comparison.
And as you demonstrate I could have tested it easily enough to confirm I was reading the code correctly...
https://cs.opensource.google/go/go/+/refs/tags/go1.24.3:src/...