The lack of a mark in box 13 may or may not be a mistake depending on the fiscal or calendar year adopted by the 401(k) plan. If the addition to your 401(k) was made for the 401(k) plan year ending in 2021, you are a covered individual for 2021 and box 13 Retirement plan on your W-2 should have been marked.
The fact that you made no contributions yourself does not determine whether you are a covered individual. The employer contribution makes you a covered individual with the only question being the year for which that employer contribution makes you a covered individual.
