I think I dug into that recommendation a year or two ago and found that the reasoning behind was more a preference than a security thing. As long as you understand how isset() works and make sure you sanitize all input, whichever way you want to do it is fine. I wouldn’t say one is a slam dunk over the other.
Even if you do use the REQUEST_METHOD trick, you still have to check that individual post field to see if it is set.