Thanks for point it out. A developer told me that this code snippet was sufficient to conclude that the ACPI wakeup was broken, so I misinterpreted it to be "the method was not defined" (while totally ignored that any AML code must be executed by the OS). Just RTFMed quickly, hopefully I've reached the correct understanding: "_PRW" is a definition of wakeup capability of this power button, the first argument "0x00" means its event is located at register GPE0, bit 0, and the second argument "0x00" means it's capable of waking up the system from ACPI S0 ("power on"), which excluded all low-power states like S3 or S4 - which was probably why I was told that S3 wakeup was broken.
Hint: to execute any code, including quoted ASL, the processors has to be woken up and running in a proper context.