It is not correct that the people of the United States, relative to comparable countries, are the most lightly taxed.
Conclusion
It is not correct that people of the United States are the most lightly taxed of comparable countries.
Evidence
Although the U.S. has the lowest tax-as-percent-of-GDP among Western industrialized countries, Americans pay out of pocket for goods/services that other countries provide via tax revenues. Health care is an example: U.S. private health spending is about 7% of GDP. The argument then says "this 7 percent, then, amounts to a tax."
Evaluate
The gripe we're supposed to have with this argument is with the last sentence. What do you mean the money we privately spend on our healthcare is a tax?
A tax, by ordinary definition, is a compulsory payment to a government. Private health-care spending is not paid to a government and is not legally compelled in the same way.
If other countries provide housing for their citizens, does that mean that money we in the U.S. spend on rent/mortgage is a tax?
Of course not. Our rent is a payment to the landlord. Our medical expenditures are payments to a doctor or insurance company or to CVS for our prescriptions. A tax is a payment to the government.
Goal
Find an answer that names the move of stretching the term "tax" beyond its proper application.