Agent forwarding is tremendously useful, but you should not forward your agent to systems you don't control or can't trust. If you do, you risk having your agent hijacked and used to connect to any system where you have public key authentication enabled.

The author implies that you need to enable agent forwarding to do a git pull from GitHub, but that would be incredibly dangerous. This can't possibly be true, can it? If so, you might as well hand them your private key and passphrase.