The introduction to the Fortune 500 states:
In total, Fortune 500 companies represent two-thirds of the U.S. GDP with $13.7 trillion in revenues.
I checked, and this is indeed the sum of those 500 companies' individual revenues.
However, I'm wondering how meaningful this statistic is. Some of these companies will sell goods to each other (for example Apple to Walmart), so I would expect double-counting of those sales when taking the sum of all individual company revenues.
How should I correctly and soundly interpret an aggregate of individual revenues and use it to assess the combined 'impact' of the companies?