To prove that I was right with D70 and SB800 FP operation at 1/4000 and my understanding was correct about this sharade, I performed test with D70 set in manual at 1/4000. On purpose I used a non-FP flash, notably great flash Sunpak 555
, and expected partially exposed frames.
Using non-FP flash at 1/4000 I expected to obtain partially exposed frames, but to my surprise ! entire frames were exposed correctly, end the recorded picture data stated 1/4000 shutter speed. I just could not believe my eyes. The only explanation is that Sunpak 555 flash slowest duration is 1/200 and that is certainly long enough to cover narrow shutter slot (1/4000) travelling across the sensor that only needs 1/500 sec to complete the journey.
The bottom line is that D70 shutter speed faster than 1/500 can be used with FP flash, or any flash with longer duration than 1/500 sec, but results are not quaranteed by anyone. You are responsible for setting appropriate flash output level to obtain correct exposure.