are all 18650 batteries interchangeable with one another?
Even though they are not interchangeable, most likely you could use a wide variety of 18650 batteries with that light. The manual is BS as far as I am concerned. I notice that the spec sheet says even in turbo mode the 3.2 Ah battery will last for over 2 hours. This means it is actually not a high discharge application. The cell is only running at 0.5C or less.
So you can use any quality 18650 cell. Unfortunately there are a lot of junky cells out there so buyer beware. For sure anything that claims more than around 3.5 Ah is fake or junky.
I would suggest getting a protected cell rather than a raw/bare cell.