Not all power leakage is through power consumption by the card. The USB-A is dumb, but it also causes a noticeable increase in power usage by the laptop (even in idle). Apparently this is due to some connection that a USB-C-to-USB-A adaptor makes between certain pins of the USB-C that triggers the USB-C interface to do some discovery about possible PD requirements, and this then uses a noticeable amount of power. Since laptops with native USB-A ports don’t tend to have this problem with those, I guess it means that USB-A by itself can be wired in a way that doesn’t cause this draw.
I guess the USB-A card should really be a little less dumb and detect somehow if there is anything plugged in (through a mechanical switch maybe?) and then switch the connection on the USB-C side depending on that.
The expansion cards should really be marketed as well-fitting dongles.