I think the misunderstanding that many people have is that if the participant did not fill out a beneficiary form/designation, then the account is subject to the terms of a will, or if no will, then intestate rules.
It isn't. 401(k) plans have default beneficiaries written into the governing plan documents, so that in the event a participant passes without a affirmative beneficiary designation, there is a default beneficiary.
Typically that is something like spouse, children, estate, but it varies. Read the plan's document carefully.
Even if the estate is where the benefits are to go - they go there because of the beneficiary rules in the plan document, not because of the application of a will or intestate laws. So If everyone else pre-deceases the participant (not what we have in this post) the estate is the named default beneficiary under the terms of the plan, and gets the $$ because of that.