Is there any way to install erpnext offline without using GitHub clone?
TL;DR - Not easily. Because the Bench commands have hard-coded calls to http://github.com.
I submitted a pull request 2 months ago that mostly solves this. Instead of hard-coded URLs, you can configure how Bench fetches repositories. You could point at a local repository source, and never use the Internet.
However, this pull request has not been reviewed and approved.