I was unaware of this claim but you may be tight.Nothing to do with scaling. From 24 core Max to 32 core Max it doesn't scale by 33% either. It's about GB's flawed results in GPU compute.
GFXBench does give much better scaling from M1 Pro to Max (usually almost 2x as expected).
Interestingly GFXBench does show frequent scaling problems from Max to Ultra.

GFXBench - Unified cross-platform 3D graphics benchmark database
The first unified cross-platform 3D graphics benchmark database for comparing Android, iOS, Windows 8, Windows Phone 8 and Windows RT capable devices based on graphics processing power.

GFXBench - Unified cross-platform 3D graphics benchmark database
The first unified cross-platform 3D graphics benchmark database for comparing Android, iOS, Windows 8, Windows Phone 8 and Windows RT capable devices based on graphics processing power.

GFXBench - Unified cross-platform 3D graphics benchmark database
The first unified cross-platform 3D graphics benchmark database for comparing Android, iOS, Windows 8, Windows Phone 8 and Windows RT capable devices based on graphics processing power.
Of course these are not quite comparable in that GFXBench is comparing graphics, whereas GB5 is comparing GPU compute, but for now I'm convinced that there does seem to be a problem with GB5 compute (at least for the particular cases of Max and Ultra).
On the other hand, it does seem fair to conclude that the scaling problems from Max to Ultra are legit, not just a GB5 issue.