Because you were accruing service time used in calculating any potential future benefit from the plan, you are considered to be covered by the pension plan even if you are not yet vested in the plan. From the box 13 instructions with regard to defined benefit plans (which is what a pension plan is):
Employee qualifies for employer funding into the plan, due to age/years of service—even though the employee may not be vested or ever collect benefits
and requires that box 13 Retirement plan be marked, indicating participation in a workplace retirement plan.
