I currently speak and understand English, Spanish, Cantonese and Mandarin to varying degrees.
I was forced to take French for 2 years in high school. Never even took it seriously. But because of that, after 6 weeks of private Spanish tutoring, I was able to hold hour+ long conversations with strangers while backpacking LatAm.
I've spoken Cantonese my entire life (but not truly native level). I took an entire year of college level accelerated "Mandarin for Other Chinese Language Speakers". I took it quite seriously. I'm backpacking China right now. I still can't even talk to anyone for more than a couple minutes without having to use a translator or look up words.
> there are plenty of people in Guangzhou who are perfectly bilingual
There are also plenty who move to Guangzhou and Shenzhen and can't pick up Cantonese at all. Turns out having an authoritarian government force Mandarin on you will make the Cantonese speakers bilingual rather quickly.