From the IRS guidance on Are You Covered by an Employer's Retirement Plan? (bold emphasis mine):
You’re covered by an employer retirement plan for a tax year if your employer (or your spouse’s employer) has a:
Defined contribution plan (profit-sharing, 401(k), stock bonus and money purchase pension plan) and any contributions or forfeitures were allocated to your account for the plan year ending with or within the tax year;
IRA-based plan (SEP, SARSEP or SIMPLE IRA plan) and you had an amount contributed to your IRA for the plan year that ends with or within the tax year; or
Defined benefit plan (pension plan that pays a retirement benefit spelled out in the plan) and you are eligible to participate for the plan year ending with or within the tax year.
Also, from General Instructions for
Forms W-2 and W-3: Form W-2 Box 13 Retirement Plan Checkbox Decision Chart:
Type of Plan: Defined contribution plan (for example, a 401(k) or
403(b) plan, a Roth 401(k) or 403(b) account, but not a
457 plan)
Conditions: Employee is eligible to contribute but does not elect to
contribute any money in this tax year
Check Retirement Plan Box? No
Conditions: Employee is eligible to contribute but does not elect to
contribute any money in this tax year, but the employer
does contribute funds
Check Retirement Plan Box? Yes
Conditions: Employee contributed in past years but not during the
current tax year under report
Check Retirement Plan Box? No (even if the account value grows due to gains in the
investments)
So it looks like there's two conditions: (1) a defined plan in place AND (2) contributions made to one's account to be "covered by the plan". In my case the second condition wasn't met so I wouldn't be "covered by the plan" even though it's still open.