They don't get the same access.
As a result of the backlash, Nvidia changed its policy to allow game developers access to the source code as of mid April of last year. However according to AMD, this did not alter the situation, as game developers engaged in the Nvidia GameWorks program were still not allowed to work with AMD to optimize the code for their hardware. Something which Nvidia initially denied but later Nvidia’s Tom Petersen and Rev Lebaradian admitted. Witcher 3 developers, CD Projekt Red, reaffirmed this again two days ago.
but even so the source code isn't needed by the developer they just need a recent build to make the drivers work, it's how the vast majority of game drivers are created as the "gold" copies are out in some cases weeks before the game actually releases, it's the same as early review copies are often floating about, if a company the size of AMD can't get hold of these early copies and make a driver before release it is a problem.
but even then the game has been out best part of a week now, there is no excuse for AMd to still be working on the drivers, when Nvidia has a problem with drivers they generally fix it within a matter of hours.