I won't believe it until you've tried it yourself and told me it works. I've heard so many different methods of catching Mew with procedures similar to that one. Although RBY were pretty buggy, with that Missingno. and rare candy trick also.
But I have a GameShark and with that I can catch it at any level and teach it any moves I want. I love my Magikarp that knows FireBlast and Fly!
But, those days are long gone, I could never go back to RBY now that R&S are out.