You can use Form W-4P to reduce the withholding rate if you had previously requested withholding in excess of 20%, but you cannot reduce the withholding to below 20% on the amount that is eligible for rollover.
For portions of a distribution that are not eligible for rollover such as an RMD, the default (not mandatory) withholding rate is 10% and you can use Form W-4P to specify a withholding rate of either 0% or more than 10% on that portion. This is likely what the last sentence from the T. Rowe Price response is referring to.
