Even though they are called exactly the same thing, a GitHub pull request and a ‘git request-pull’ are completely different.
The git request-pull is for generating a summary of pending changes to be sent to a mailing list. It has no integration by default with GitHub.
The GitHub Pull Requests is a fully featured function of GitHub only. It allows for merging and integration of code from a different branch/fork. You can resolve merge conflicts, do code reviews, or add additional comments to a GitHub pull request.
Unfortunately the git command is named similarly to GitHub functionality which makes it sound like they should be doing the same thing.