Answering my own questions again…
The answer is “Domain Users” is not actually in a user’s memberOf attribute.
As silly as it sounds, it’s because Domain Users is not actually in the memberOf attribute. You can verify in ADUC by turning on View - Advanced Features, going to the Attributes tab on your object and opening the memberOf attribute (not the “Member Of” tab).
The “Member Of” tab you see on an object’s properties in ADUC is actually a conglomeration of the memberOf attribute and the primaryGroupID attribute. By default, users in AD get their Domain Users membership via this primaryGroupID attribute rather than an entry in memberOf. Though it’s possible to change the primaryGroupID, most people don’t.
The “memberOf” attribute itself isn’t “real” either… AD stores membership information in the groups’ “member” attribute, “memberOf” is calculated from that.