Your link is talking about Market concentration in terms of total US imports and exports. It's possible for the HHI to be low in global terms if the US is impor

Your link is talking about Market concentration in terms of total US imports and exports. It's possible for the HHI to be low in global terms if the US is importing/exporting to a variety of partners even if the production of goods is concentrated in few hands. So yes, this measure of market concentration has been going down worldwide as globalization increased partnerships between countries, but in this case the HHI index you're referring to isn't measuring microeconomic domestic competition, it's talking about international competition on a macro scale, treating countries as the inputs.