The group are not employees. They're ex-employees. There is no employer relationship in this case. The responsibility for withholding is somewhere else.

I would be wary of assuming that is true for purposes of RSUs. Whether these folks and the company consider them current "employees" is not the employee-employer test for tax withholding purposes.

I'm no expert - not even a talented amateur. But, RSU's for employees are equity, and RSU's for non-employees are expense. I'm assuming, because books must balance, that there's a similar split in tax (and responsibility for tax).

RSUs are wage income, and subject to supplemental wage withholding. The responsibility for wage withholding is with the payer.