That is quite true. The point was that even though the car is charged at the cheap rate the increase in the day rate offsets to varying degree the night rate so the real cost of charging and running your EV is the average rate you are charged. Many people either don't understand this or won't accept this and continue to claim their EV only costs the night rate to run. This logic only really applies when the day rate is increased as part of the EV tariff.
The cost of charging is not the average rate of what you are charged overall as the sample sizes (kWh used) are rarely the same at the different rates.
You are effectively trying to average averages, even though you don't realise it, which doesn't work in a straightforward way. (the average price of daytime use is 41p the average price of off peak is 12p in varying sample sizes of 50 and 100 in my example below)
For an example in a given period your
daytime use is 50kWh at 41p total cost £21.50
charging at cheap rate 100kWh at 12p total cost £12.
The simple blended average would be total price / kWh = 21.66p at these prices for this sample size and will be different for anything else.
However we have a base we can use that is the cost of SVR electricity at 34p to create a better way of calculating the true cost of charging.
So in the above example at 41p we have overpaid by 7p for 50kWh compared to SVR total £3.50
If we add that to the cost of charging which was £12 gives us £15.50.
Now divide that by the 100kWh we charged and the actual cost of charging is now 15.5p compared to if we had been on a SVR.
This also highlights that the OVO offering of actually being on the SVR and adding the free anytime charging at 10p is an excellent offer, currently, if you can access it.
NB All figures will change based on differing usages and would need to be calculated each time but this is just an example
SVR - Standard Variable Rate