Who knows. Maybe he's already tried using ATMOS in his movies and he thought the SQ was even worst than this DTS-HD MA.
Sound quality is a bit of weird way to term this.
DTS-MA is 8 streams of full 8 24b48kHz lossless audio. It's basically the same as Atmos. The difference being that it is hard mixed and doesn't have the spatial information of Atmos, so you don't get the overt overhead data. I know there's a little bit more going on with the data other than heights but my point is that Atmos isn't any more clear or higher resolution in fidelity. So I don't really see this as a quality issue. More of a surround-sound presentation difference.
For what it's worth, Atmos height is rarely used to really stand-out effect. Most of the time when I let my processor just upmix into the heights, I notice very little difference between it and DTS-MA 7.1 while actually watching. Maybe I might more if I was only paying attention to the sound but I don't really get fussed about a good DTS-MA mix instead of Atmos.
I'm not really trying to defend his "stuck in the past" stance. But I really don't see it as a huge "quality" hit on his films either.
