Most understand Mandarin. Many speak it. Quite a few of them speak it badly.
A Hong Kong guy who used to work for me thought he was a fluent Mandarin speaker. So he insisted on doing most of the talking whenever we had client meetings in China.
The problem was that when he spoke Mandarin, it just sounded like very weirdly accented Cantonese. I knew that because I speak Cantonese, but my Mandarin is atrocious. Yet somehow I could understand everything he was saying (it wasn’t just me btw. My China team agreed that his Mandarin was in need of improvement).
Not all Hong Kongers are like that. I know some who speak Mandarin very, very well. But standards vary widely in the territory.