You're both correct, but USB Power Delivery, like so many things USB, is a total mess with different revisions, and versions of those revisions, that each extend and deprecate parts of the previous revision. I'm going to ignore the actual protocol used for negotiation here, but there have also been at least three different protocols.
The first revision, rev 1.0, had six fixed power profiles: 5V/2A, 12V/1.5A, 12V/3.0A, 12V/5.0A, 20V/3.0A and 20V/5.0A. These were deprecated by rev 2.0, version 1.2, which instead introduced power rules at four fixed voltages, supporting power supplies with different output powers. The maximum supported currents by the specification were 5V/3A, 9V/3A, 15V/3A and 20V/5A. These power rules were retroactively named the Standard Power Range (SPR) in rev 3.1. That revision also added the Extended Power Range (EPR), which raised all the current limits up to 5A, but only when used with electronically marked cables. EPR also added power rules at 28V, 36V and 48V, again requiring (differently) marked cables and up to 5A. So 5V/5A is a valid option, but only when used with a USB PD rev 3.1 compatible power supply and cable.
Orthogonally to the fixed power rules, rev 3.0 introduced the Programmable Power Supply, which allows a configurable voltage between 3.3V and 21V in steps of 20 mV. This was extended by rev 3.1 with voltages between 15V and 48V in steps of 100 mV, called the Adjustable Voltage Supply. As far as I'm aware this is not commonly used, and most products advertising Power Delivery support use the predefined power rules.