Yeah how can they say it has the “fewest bugs any Bethesda game has shipped with” when the game hasn’t shipped yet??

Issue tracking has been a part of software development since the beginning. They know and have always known roughly how many bugs they have shipped games with. Just like any company that releases a product knows roughly how many bugs they are shipping with. I pretty much guarantee you that any software that has ever been released has had a huge backlog of bugs of varying levels of importance sitting on some form of backlog.

So, it's pretty straightforward for them to know how this game is comparing against their previous releases. Not to say that there won't be plenty of bugs that have been missed, but that's not really the point.