I think it is the employer's screwup. The employer should not have checked Box 13. From the IRS:
https://www.irs.gov/Retirement-Plans/Are-You-Covered-by-an-Employer's-Retirement-Plan%3FYou’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.
Box 13 on the Form W-2 you receive from your employer should contain a check in the “Retirement plan” box if you are covered. If you are still not certain, check with your (or your spouse’s) employer.
Furthermore (also from the IRS):
https://www.irs.gov/Retirement-Plans/Common-Errors-on-Form-W2-Codes-for-Retirement-PlansForm W-2, Box 13
The “Retirement plan” indicator in Box 13 shows whether an employee is an active participant in your company’s plan. If this box is checked, it lets the recipient know that depending on their filing status and modified adjusted gross income, they may not be entitled to a full deduction for their traditional IRA contributions. You should check the retirement plan box if an employee was an “active participant” for any part of the year in:
a qualified pension, profit-sharing, or stock-bonus plan under Internal Revenue Code Section 401(a) (including a 401(k) plan).
an annuity plan under IRC Section 403(a).
an annuity contract or custodial account under IRC Section 403(b).
a simplified employee pension (SEP) under IRC Section 408(k).
a SIMPLE retirement account under IRC Section 408(p).
a trust described in IRC Section 501(c)(18).
a plan for federal, state, or local government employees or by an agency or instrumentality thereof (other than a 457(b) plan).
Active participant
Generally, an employee is an active participant if covered by a:
- defined
contribution plan (for example, a 401(k) plan) for any tax year and is
credited with any contributions or forfeitures, or
- defined benefit plan for any tax year that the employee is eligible to participate