Yes, that's exactly what that means. And no, that doesn't make any sense. And yes, you need to restore the forfeitures (plus earnings).
The IRS believes that employees who terminate during the applicable period must be 100% vested, even if that termination was voluntary. They state the following in their online FAQ (linked below): "An affected employee in a partial termination is generally anyone who left employment for any reason during the plan year in which the partial termination occurred and who still has an account balance under the plan."
https://www.irs.gov/retirement-plans/retirement-plan-faqs-regarding-partial-plan-termination
It doesn't make sense that voluntarily terminated employees would have to be fully vested due to a partial plan termination. That seems at odds with the intent of the partial plan termination rules, especially since you don't count such employees to determine whether there has been a partial plan termination in the first place (see the following, excerpted from the exact same IRS FAQ linked above):
"Do employees who voluntarily quit count for purposes of determining a partial termination?
Generally, voluntary terminations do not count in determining whether a partial termination has occurred as they do for determining who must vest after partial termination. However, in some cases employees who appear to terminate employment voluntarily have been found to have terminated involuntarily under a constructive discharge theory. The employer's intent, working conditions and the reasonably foreseeable impact of the employer's conduct on the employees are factors in evaluating a constructive discharge."
IRS Revenue Ruling 2007-43 states, "If a partial termination occurs on account of turnover during an applicable period, all participating employees who had a severance from employment during the period must be fully vested in their accrued benefits, to the extent funded on that date, or in the amounts credited to their accounts."
The IRS is actively enforcing this position. See the 2016-11 issue of Employee Plans News (linked below) describing the EPCU project that contacted over 2,000 Form 5500 filers asking questions about potential partial plan terminations:
"In almost 10% of the cases, it was determined during the compliance check that a partial termination had occurred and affected participants had not been fully vested. In most of these cases forfeitures were reinstated. In a few of these cases, it was not necessary to reinstate the forfeitures for some or all of the affected employees because no distributions had as yet been made. In these cases the vesting percentages were corrected. The amount of forfeitures plus interest that was reinstated for cases with forfeitures totaled $2,208,678."
https://www.irs.gov/retirement-plans/employee-plans-compliance-unit-epcu-completed-projects-with-summary-reports-partial-termination-partial-vesting