I think you may have missed the point a bit, at least the way I read it.
In the article, some top name professional players discuss how much other sports helped them with various aspects of their athletic make up. Nothing new there. The difference is the research - that numbers show, in summary, that the average age that HS athletes started specializing is about 12 1/2, the average age for college athletes is almost 15 and the average for pro's is over 14. So, on the surface, one message is that you don't have to specialize TOO EARLY to make it to the next level.
Yes, you are right about the bias. In part, pro athletes don't have to specialize as early because they were great athletes and were able to make the teams of other sports and able to survive missing some team practice time and extra game experience due to overlap. But that doesn't completely take away from the point that, if you have the abilities within you to play at the college level and beyond, you don't need to specialize too early.
However, what is happening in so many places is parents and VERY young players are succumbing to pressures to specialize early... like pre-HS (9-12 or so, sometimes even younger). Sometimes WAY before HS. This is where the problem lies in the minds of most. So, putting these specific numbers on it helps with perspective.
In the article, it is stated that yes, you will have to specialize at some point. But the numbers shed specific some light on what age that is likely to be.
I don't see the same never-ending stream of articles questioning the HS players playing just one sport. It seems to me that most debate is in regards to pre-HS ages. Once in HS, there are true difficult decisions to be made by many and many others have that decision made for them.
Just my opinion, experience and interpretation.