It's the update that is incorrect, the original article is fine. The update fails to recognize that after tax sub accounts can be treated as a separate contract. Here is Tax code Sec 72(d)(2):
72(d)(2) Treatment of employee contributions under defined
For purposes of this section, employee contributions (and any
income allocable thereto) under a defined contribution plan may
be treated as a separate contract.
To be considered a "separate account", the plan must maintain separate accounting for the employee contributions (pre tax deferrals and co matching are both technically employER contributions). With such separate accounting, distributions from the separate account are NOT pro rated with the other plan balance.
There also exists a "grandfathered" plan balance for pre 1987 after tax contributions in the plan. If separately tracked, the balance of pre 87 after tax contributions can be distributed by itself, separate from even the earnings on those contributions.