According to the rule right before the rule you quoted:

Path patterns take precedence over identifier patterns.

Your example attempts to match the constant a rather than binding it as variable. If you did something like

const a: () = ();
fn main() {
    let a = ();
}

it would have worked with a warning as now the constant a infallibly matches against the value (). It still doesn't shadow the constant.

warning: constant in pattern `a` should have an upper case name
 --> src/main.rs:3:9
  |
3 |     let a = ();
  |         ^ help: convert the identifier to upper case: `A`

@bjorn3 I understand it now. Thank you.