Do you have any reason to suspect GitHub isn't blocking those countries? How long do you think an offensive-security sponsor/passport-issuing nation might take to get around GitHub IP-blocks?

Right exactly. The only way IP blocks work is if there's no vulnerable machines to take over anywhere. That is - it basically doesn't work for any motivated attacker.

You could hypothetically make it work, but it would mean an extremely different Internet and device landscape than exists today. (And even then I doubt it stops a nation-state level attacker, they can always use old fashioned espionage to get someone in meat space and get around any technical barrier)