So I did a cursory inspection, and they are very similar. The exhaust system is composed of three sections, the center resonator with attached pipes, the left muffler and connecting tubes, and the right side with connecting tubes.
The left and right muffler sections are identical in part numbers for the V8. The resonator is where things aren't identical. The part numbers are extremely similar, but not the same, denoting some change made between the two. I can say that just going off of the picture, they look size wise to be able to fit in the same place.
here is the website if you want to take a look for yourself -
Hyundai Parts - Genuine OEM Hyundai Parts and Accessories Online
edit-One other thing I can tell you is that, after looking at the frame stamping part numbers, the V8 uses the same center floor stamping p/n-65111-B1040 (which is where the resonator and associated plumbing lives).
I 'think' Magnaflow's 2016 exhaust will work on a 2018, and the reason they are probably saying no is due to not testing the fit on the 2018.