EOS R HSS question

Went to the corn maze with the kids a few days ago, and brought the R and a flash. However, when trying to use the flash for fill in HSS mode, the resulting photos were overexposed. I then looked at the manual, and the manual states "High speed sync is not available in [Av] or [P] mode when set to [1/200 sec. (fixed)]". Well, I had been in Av and my flash was set to 1/200 sec. fixed. So the performance is consistent with what the manual states, but the question is why? If HSS is for cases where the shutter speed is faster than the sync speed, why should "slow synchro" be set to a slower option to enable HSS?