Finally I had to contact the technical support. They saw my remote repository URL like https://github.com/Owner/Repository and told me to replace it with something like email@example.com:Owner/Repository.git! All cleared.
These documents and answers all assume that you are familiar with the SSH-Git workflow and nobody has ever mentioned about the URL difference between SSH and HTTPS. I always thought a HTTPS URL + a SSH key are all I need to access the remote repository without username and password. Nowhere in all these did I see any serious instruction that a special SSH URL is required.
I post my experience here, hoping someone like me will see this and quickly get his job done. However, I also strongly suggest the MATLAB official to update their document on this. If it's too wordy to talk about SSH-Git workflow in the MATLAB document, at least I want a hint or a reference so that followers won't make similar mistakes and waste so much time confusing.
The newly prereleased version R2021a now automatically remembers your username and password for HTTPS! Now you just need to input those stuff only once, and don't have to deal with those tricky SSH shit. Well done!
Now the best advice for newcomers suffering from old versions is: update to R2021a!