How can the deaf guy speak japanese if he was born deaf? Sorry if I'm being stupid

I'm answering because I think you meant well.
Usually, deaf people who are able to speak are the ones who lost their hearing or they're not completely deaf (it's an spectrum)
BUT!
There are ways to learn how to pass air through your throat and create sound. Then, you learn how to shape those sounds, and a teacher coaches you about that, and teaches you how to speak. It demands a lot of effort, so some people born deaf don't always do it. And yes, they usually have a weird accent. They can't adjust their accent, because they can't hear it. So if they started learning to speak as a child, it's possible to give up because of bullying.
The deaf community is very protective and sometimes they don't bother reaching out to otsiders.
And also, each country has it's own sigh language. If you learn asl (american sign language), it's possible you won't be able to communicate with a person that only communicate with jsl (japanese one.)

Thank you sm for your answer!