Any good DC lab power supply can operate as a constant voltage source or constant current source. By default, the power supply will maintain a constant voltage at the terminals, but if the connected device starts too pull a current that's higher than the specified limit, the power supply will lower the voltage to keep the current in check.
Not saying this is the case here, of course, but they could do something similar that way if they wanted to...
These are not DC power supplies, thats whats inside the car, a boost DC power supply converting 240v AC to 400v or 800v DC, thats what regulates current and voltage to the batteries.
If the AC "charger" attempted to regulate current by varying voltage the actual onboard charger would just attempt to pull more current to make up for the voltage drop and probably trip the whole thing.
There would be no point in regulating the AC current, it would make the charger very complicated and expensive replicating much of whats in the DC converter/charger in the car.
A full DC charger like at a charging station does do this because it is the actual charger bypassing the cars small internal DC charger with DC leads going directly to battery at full pack voltage.
Not saying this is the case here, of course, but they could do something similar that way if they wanted to...