If a frozen DB plan is top-heavy it must provide top-heavy minimum benefit accruals to all non-key employees unless no key or former key employee benefits under the plan during the plan year. An employee "benefits" if the employee has an increase in a benefit accrued or treated as an accrued benefit under IRC §411(d)(6). See IRC §416(c)(1)(C)(iii). Note that this exception does not only apply to a frozen plan but applies to any plan under which no key or former key employee benefits during a plan year, including DC plans.
However, be sure to check to see if the employer also sponsors another DB plan or a DC plan because these plans may form a top-heavy aggregation group. If the top-heavy aggregation group is top-heavy, top-heavy minimums will still need to be provided within the group. See Regulation §1.416-1 T-6, T-9 and T-10.