startTransition marks updates as non-urgent

What does this output?

const log = [];

// Simulates React's startTransition
let isInTransition = false;
function startTransition(fn) {
  isInTransition = true;
  fn();
  isInTransition = false;
}

function setState(value, label) {
  if (isInTransition) {
    // Non-urgent: can be interrupted
    log.push([transition] ${label}=${value});
  } else {
    // Urgent: runs immediately
    log.push([urgent] ${label}=${value});
  }
}

// Urgent update (typing in input)
setState('a', 'query');

// Non-urgent update (filtering 10k items)
startTransition(() => {
  setState([1, 2, 3], 'results');
});

// Another urgent update interrupts
setState('ab', 'query');

console.log(log[0]);
console.log(log[1]);
console.log(log[2]);

Correct Output

[urgent] query=a
[transition] results=1,2,3
[urgent] query=ab

Why this output?