This is journalistic misconduct (on the part of the Washington post), imo.

Given UHG’s size and broad reach – “more than 5 percent of U.S. gross domestic product flows through the company’s systems every day”

That’s impossible. It would only be possible if there were only 20 business days a year and UHG was the only company. This article sources it (thankfully) to the Washington post, who source it from here, where it’s much more reasonable:

It is this analyst’s opinion that the concentration of ownership of health care payment infrastructure in the hands of a single company poses a significant threat to the financial viability of the US health care system and requires a policy response. It does not make national security sense for more than 5 percent of US GDP to flow through a single company’s pipes. No provider of administrative services to health plans or care systems should be permitted to control a third of total US health care payments. Assuring the stability and safety of this payment infrastructure is a legitimate and substantial policy challenge.

UGH is a $1,5T company and the US GDP is around $30T. 5% is reasonable to say, but saying 5% every day is wrong.