A charger capable of 5A and 20V is the "most powerful" charger. But some devices don't work at 20V.

A charger must be able to supply 5V, and may be able to supply 9V, 15V or 20V. It might supply all 4, or some subset. All devices must support 5V and may support other voltages. So if someone makes a charger that does 5V and 20V and then you plug in a device that wants either 5V or 9V then they will agree on 5V. Then that means you can't charge faster than 15W.

According to the spec it should still charge, just charge slowly. But some companies interpret the spec rather liberally or just don't comply. And then it may not charge at all.

Honestly, Valve seems to do a rather poor job with USB C (USB-PD) really support. So their charger is probably poor at it. A good quality USB C charger may support both your headphones and Steam deck. In case you care, like if you wanted to take only one charger on a trip. Better test it first before you leave on the trip of course!