8k video

Yes. From what I've seen in the reviews this is the only option. None of them mentioned transferring it after the fact so that may be an option. The reason for this limitation is the speed of the write rate. SD cards are not created equal... This is why you can see widely varying prices for the same sized storage. What separates them is read write speeds and you pay a premium for those faster speeds. I could speculate as to why they locked it to the internal memory but it's probably just due to write speed.

Check your card and compare it to this list and see how it fairs for speed... https://www.sdcard.org/developers/overview/speed_class/
 
Sd cards are slow, especially compared to the internal storage. High end cameras don't use SD cards for this reason.

If the storage isn't fast enough it won't record the video properly, there will likely be artifacts and dropped frames.