What the holy hell are you talking about? I think you're getting watts and volts confused. 20V/14A is 280w (V x A = W). EVERY AC/DC inverter takes AC, usually 120-240v and at a fairly low amperage (not sure of the exact conversion formula) and turns it into DC. In the case of my 280W supply, it's 3.2A AC.
As of now there aren't any small form factor 280W bricks but with GaN making advances rapidly it's a matter of time.
They're referring to the fact that the new USB EPR standard capable of delivering 240W, as discussed in this article, does so at 48V. So your laptop is expecting 20V from your power supply, but you'd be supplying it with 48V if you connected the laptop to a USB EPR. Although I believe USB always defaults to 5V and then must negotiate a higher voltage, which the laptop barrel jack wouldn't do, so it might just stay at 5V resulting in severely reduced power delivery (unless the USB to barrel adapter is smart and can do the negotiation).
I believe laptops immediately step down and regulate the input voltage, so it can handle some variation in input voltage (e.g. the same laptop can run off an ~11V battery or a ~20V power supply), but I wouldn't necessarily assume it can handle 48V.
As an aside, "inverter" refers to a DC to AC converter. Going the other way is called a rectifier (or just AC to DC converter).