[SuperNinKenDo]

I agree that counter evidence to the claim, but one thing to keep in mind is that Cantonese is a local language, which many speak at home and with close friends and family from birth (or at least some closely related dialect), while Mandarin is often not. So if you concede that something like Pinyin might help L2 learners, it's reasonable to think it might be less necessary, but still helpful in an L1 setting, which Mandarin sits somewhere between, and Cantonese is well in to L1. It will be interesting to see if something like Jyutping takes off in the native Cantonese speaking community at some point or not. It's also a different case because spoken and written Cantonese are such different languages, although I don't want to overstate what effect, if any, that might have.