Checking my tables, I find that the formula is OK for WB temps >64 deg, indoor DB around 80 and ambient temps > 80 which makes it a rule of thumb formula.
Since it doesn't take account of indoor DB it is especially limited.
Since air at 64°F wet bulb has the same BTU at 74°F DB, that it does at 80°F DB, or 90°F DB ( 29.31 BTUs per pound of air, at sea level).
What allowance do you think it needs for indoor dry bulb. Its the same load.
Well since my charts show at 64 WB and outdoor ambient = 85 deg a range of 8 to 14 degrees SH for indoor DB range from 70 to 90 deg DB, The enthalpy of the 90 deg air and the 70 deg air differ by the sensible heat , so WB is close but not an exact indicator of enthalpy.
Upon looking into this more, it is clear, as you say, that the enthalpy is practically independent of DB and virtually depends on WB alone.
That, however doesn't say that the same input (WB) enthalpy into the A coil yields the same SH, since as you know at 90 the increase in sensible heat is offset by the decrease in latent vs 70 deg and maybe more heat is transferred at 90 deg since the average delta T is between the coil and the air is higher. However, more may be at play here.
Just my $.02.
At 90° indoor temp, and 64°WB, the RH is much lower, so you do have a much lower SH. Because you are removing more sensible heat. Then at 80° ID DB and 64° WB. At the same outdoor temps.
But chance the OD drybulb, and the Target SH jumps.
ID Wb of 64.
At 90° OD, 11°SH.
At 80° OD, 16°SH.
At 70° OD, 21°SH.
Manufactorurer specific charts may show slightly higher or lower. But will have a =or- tolerence.
Using the formula on one of those units, is no different then using another brands slide chart on it.