They're not interchangeable. USB-C has power negotiation, but that doesn't mean the devices support those voltages. You still need to understand the voltage ratings on both ends of a cable.

That's exactly what OP is saying - chargers are commonly advertised as being able to supply a particular wattage, but that particular wattage is only attainable if the device being charged supports the maximum voltage the charger is capable of delivering.

OP is complaining that that leaves the true wattage of a given device/charger pair unknowable from the charger's packaging alone without further information as to what voltages and at what currents it can supply on request from the device. It's certainly a valid frustration.

This makes more sense and isn't how I parsed the original comment.

That is precisely the point of the original comment that you responded to.

I found this comment that explains it well:

https://news.ycombinator.com/item?id=28911240

The reverse situation would be the case (cable supports 3A, but not 6A). For the range of voltages supported by USB-PD, the cable only cares about amperage. Copper wire doesn’t care much if it’s 5V, 50V, or 250V, it’ll carry it the same. However, if the size of the wire isn’t correct for the amperage, it’ll over heat and potentially catch fire.

It’s the end device that cares about voltage and amperage both as the voltage steppers inside have to be ready to handle providing the right voltage to the chips and the amperage has to be right to handle the load.