Hong Kong has been teaching Cantonese Chinese without using a romanisation system to do so and its literacy rate never lagged.
Pinyin may help second language learners greatly, but was never the cause of the literacy spike. It's the rollout of the education system to villages that caused it. If they taught Chinese characters without Pinyin, they would most likely have decreased literacy rates to a similar extent also.
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.
Pinyin may help second language learners greatly, but was never the cause of the literacy spike. It's the rollout of the education system to villages that caused it. If they taught Chinese characters without Pinyin, they would most likely have decreased literacy rates to a similar extent also.