"and the taxpayer was unaware that the ex employer was withholding on his wages at one rate and his severance at 22%."

It's prepayment against actual taxes that will be owed from the 1040. If they don't owe that, they get a refund. If they owe more, then lucky for them, some of it is already paid in. As I mentioned, there are three ways to compute it.

"He is underpaid by a substantial amount of money and he contacted his employer to understand what I just described. The employer told him that a) 22% is what they are required to withhold"

Yes, as long as the bonus is $1million or less.

"b) led him to believe that by attaching his paystubs to the tax return his severance would only be taxed at 22% regardless of his tax bracket."

No. Once again, that is confusing Withholding and Taxes. Did you read what I provided? Example:

His 1040 shows he has $8million taxable income from other investments, property sales, capital gains, etc. His wage is mixed into that total, the same as always. You know this; you prepare tax returns.

"I cannot find anything in a Publication or elsewhere supporting the latter."

Either the employer stated it wrong, or the employee conveyed it wrong. You know that is wrong.