I would imagine it's compounded every 8 hours or so when the rebase is received. If you compound the 8-hour rate 15 (24 hours divided by 8 * 5 days) it equals the 5-day ROI (or very close).
If you calculate the 5-day ROI rate at an annual rate you get the 8,176.6% . I just noticed that the APY is now at 8,012.5% so it seems to be constantly floating. I just calculated the 5-day ROI compounded rate and it matched the 8,012.5% current APY.
(1+'5-day ROI rate')365/5-1*100