Sure, Cantonese is spoken by 100% of people in Hong Kong and Macau. It's important if your target are these people - this usually means you're operating locally. Even in Shenzhen, it's important to speak Cantonese if your job is selling fake LVs or giving massage to customers - since your biggest consumers are Hong Kong Citizens.
But we're talking about investment banks here, right? Any Hong Kong corporation you deal with is going to know English. Which is why for the past 30 years, all your MDs and Heads based in Hong Kong are westerners and not Chinese. They never needed Cantonese to deal with Cheung Kong or Sun Hung Kai and never will.
What's changed is that investment banks now want to do business with mainland China companies like PetroChina. These companies don't speak English. Their Heads of M&A or CFO only speak Mandarin. Unlike Hong Kong corporate workers, mainland China corporate workers weren't educated in foreign institutions in America and they didn't need to be. They have been promoted internally from working as a mere engineer back in the 70s etc and as such never had the chance/exposure to learn English.
A good example would be to take a look at the Social Security Fund which is China's third largest sovereign wealth fund (which I can tell you has more cash than BHP and KKR combined). Take a look at these people's profiles. There's none of this Harvard or Yale rubbish. These are home-grown people who have went through the political system (former Tianjin Mayor is now head of it). Unlike Donald Tsang (HK CEO), Dai Xiang Long (former Tianjin Mayor and Social Security Fund Chairman) doesn't speak any English.
Another example would be to look at Morgan Stanley's Private Equity arm based in Hong Kong. They hire - but their requirements are fluent Mandarin and English. Absolutely zilch need for Cantonese.