Short answer: Not automatically. If employee contributions helped fund the plan, part of a surplus or refund may be plan assets that must be used for participants, so the employer should get advice before treating the whole check as company money.
ERISA requires fiduciaries to use plan assets for the exclusive purpose of providing benefits and paying reasonable plan expenses, and employee contributions become plan assets once they can reasonably be separated from company funds.
When a level-funded or self-funded plan returns a surplus, the question is how much of that money traces back to employee contributions. A common approach is to look at the share of total cost employees paid and use that portion for participants, for example by lowering future employee contributions or improving benefits, but the right answer depends on the plan documents and the facts, and counsel should confirm it.
Also confirm what the contract actually promises: a refund can depend on run-out claims, renewal and the carrier’s settlement terms, so it should not be spent until the amount is final.