BR,
This is where taking everything you read as gospel, and it can get you into trouble. It can be interpreted in many different ways. Common sense must prevail.
They almost certainly meant you to just take the white knuckle tightness off the holding bolts, where they still hold the chuck in full contact, but a bit of a grunt on the adjusting screws will move the chuck against the backplate.
Then after adjustment, white knuckle the holding bolts up, and recheck the runout.
That was described in laymans terms rather than technical.
But I am sure you figured that one out by yourself already.
Bogs