"Become the best at one specific thing. Become very good (top 25%) at two or more things. The first strategy is difficult to the point of near impossibility. Few people will ever play in the NBA or make a platinum album. I don’t recommend anyone even try."
ref http://pmarchive.com/guide_to_career_planning_part2.html
I don't necessarily think that's even close to impossible. You can always increase the specificity of a skill to the point where it's possible to become the best at it. If you want an example, it's near impossible for most people to become the best programmer (there can only be one), but go to github and pick a random library and pick the most active contributor to that library and you've most likely found someone who is the best at that domain. Then look at the number of libraries on GitHub. Then you'll start to see how many people are the best at a very specific domain of knowledge.
That's gold :)
Scott Adam's is slightly more pragmatic. I took his quote as meaning your chances of success are higher if you are a generalist than to pigeonhole yourself with only one skill set.
I don't think the two quotes are conflicting pieces of advice.
However, I do think it is important we consider Mr. Altman's follow up to your quote: "Even if you miss, you’ll probably end up in a pretty good place." If it weren't for this, "Aim to be the best in the world at whatever you do professionally." would have made me shift in my seat.