I understand that, but I think the target users of the Z9 would much rather have faster I/O. There are a number of articles and videos benchmarking the Z9 and the buffer performance is directly correlated to the speed of the storage. If Nikon went with Type A/SD their theoretical max buffer speed would get cut in half.
It's one of those "limited consumer choices to ensure best experience" things. Using an SD card and it's slower I/O speeds would meaningfully degrade the video and burst capability, blackout free still shooting of the camera, so might as well enforce a higher standard tk ensure a uniform experience.
A benefit of Type B is that it can host a 2230 M.2 adapter. You can take out a laptop SSD and put into a camera, almost like we all did in 200s with iPod mini and dSLR back then.