About Y rotation correction in FocusSquare.updateTransform of ARKitSampe

I thought simply setting self.rotation = SCNVector4(0, 1, 0, camera.eulerAngles.y) is enough, but not really good enough in reality. I don't know why it is not.

Nor do I undertand why the sample code does a good job. Especially why yaw is calculated using atan2f(camera.transform.columns.0.x, camera.transform.columns.1.x)?

Can someone explain the underlying math to me? Thanks.

// Correct y rotation of camera square
if let camera = camera {
    let tilt = abs(camera.eulerAngles.x)
    let threshold1: Float = .pi / 2 * 0.65
    let threshold2: Float = .pi / 2 * 0.75
    let yaw = atan2f(camera.transform.columns.0.x, camera.transform.columns.1.x)
    var angle: Float = 0
   
    switch tilt {
    case 0..<threshold1:
        angle = camera.eulerAngles.y
       
    case threshold1..<threshold2:
        let relativeInRange = abs((tilt - threshold1) / (threshold2 - threshold1))
        let normalizedY = normalize(camera.eulerAngles.y, forMinimalRotationTo: yaw)
        angle = normalizedY * (1 - relativeInRange) + yaw * relativeInRange
       
    default:
        angle = yaw
    }
   
    self.rotation = SCNVector4(0, 1, 0, angle)
}