There is a powerful technique for working out why an author thinks what they do: reading the entire article. His basic thesis can be summarised with "The short

There is a powerful technique for working out why an author thinks what they do: reading the entire article. His basic thesis can be summarised with "The short version of the story is this: GDP is a measurement of everything produced within a country’s borders. Imports are produced outside a country’s borders. So imports don’t add to or subtract from GDP. Imports simply aren’t counted in GDP at all." and he justifies it using other words. It is quite a reasonable argument - the accounting formula for GDP (C + I + G + (X − M)) is misleading because the M isn't a subtraction but an adjustment to prevent something like double-counting.