... just BS or forgetting which cam he/she used?
https://www.youtube.com/watch?v=Qc0JszHkpTk
probably was lent a 5dIII and thought it is a 5DS
AFAIK 5D3 doesn't record 4K in slow motion ...
true then either some other brand (so forgot what he/she used) or still the 5D3 with some magic post workflow