People just consistently overestimate die size measurements from pictures because the epoxy that holds the die spreads around and makes it look bigger. The difference between 80mm^2 and 74mm^2 is just 0.2mm on every edge.
Add 0.1mm-0.2 mm of scribe lines on all sides too before adding the epoxy.
Anyway, I am very doubtful of Zen3D 74mm CCD, could be a typo. Zen3 CCD when measured externally with scribe lines + epoxy is 83+mm2.
AMD mentioned 80.7 mm2 Zen3 CCD during press briefings at Zen3 launch, I doubt it will change at this point to 74mm2.
This means the MTr/mm2 improved by 10% from Zen3 to Zen3X3D, very doubtful on that considering they are reusing the CCDs for non stacked parts and the assembly and the contacts on the package has to account for this. They are not doing a new cell library and NTO just for Zen3X3D
With die shots likes the ones from Fritzchen Fritz you can account for both the epoxy and scribe lines actually, in couple of weeks someone will tear it down and we will know.