What I should have said is if 3A at a standard voltage is within the charger's watt rating, then the charger has to support at least 3A at that voltage.

A charger has a top mandated voltage that depends on its watts. Standard voltages below the top mandated voltage, which is 15V on a 30W charger, have to support 3A.

But chargers are allowed to offer higher voltages if they want to. Thank you for clarifying that.

(And I did check the spec for this post, it is mandatory, USB Power Delivery Specification Revision 2.0 Version 1.3, section 10.2.2, Normative Voltages and Currents.)

Edit: I also checked the programmable supply rules now in Revision 3.1, it looks like 1) they have the same math mandating at least 5/9/15/20 volts based on wattage, 2) you can choose any voltage between 3.3v and the mandated max, 3) at those voltages it has to supply at least min(3A, [supply wattage] ÷ [chosen voltage])