Just because a biographer had access to the subject doesn't mean the biography is necessarily better. "Authorized" biographies are notorious for slanting favorably towards their subjects, papering over their flaws and building up their good points.
Sometimes that's just a byproduct of spending a lot of time with someone; you can't spend all those hours with a person without starting to see things at least a little the same way they do. Other times it's a condition they had to agree to in order to get that access in the first place.
Which is why so much of this stuff turns on the reputation of the biographer. An authorized biography by a hack is unlikely to have much merit. The presumption is that Walter Isaacson (a) cares very much about his reputation and (b) stakes it on this biography.
You can't call this biography "slanted" without suggesting that Isaacson sold himself out. He may have, but that's a serious accusation.
I'm not calling this biography slanted -- how would I know, I haven't even read it yet -- I'm challenging the assertion that a biography written with access to the subject is automatically better than one written without it.
Sometimes that's just a byproduct of spending a lot of time with someone; you can't spend all those hours with a person without starting to see things at least a little the same way they do. Other times it's a condition they had to agree to in order to get that access in the first place.