I'm pretty sure it actually is Motif. That's how I remember it, and there's certainly enough text in the source to support that.
That's really interesting, seems like they licensed the Motif API and ported the whole thing over to DOS. Because the normal Motif is very much tied to UNIX and X.
X - sure; Unix - not do much; CDE desktop ran on OpenVMS.
It was similar to the Motif L&F but it was all implemented from scratch in the x86 assembly version.