Importing into inventories absolutely does decrease GDP. If you look at the Q1 stats some of them are normal, but imports and inventories had huge weird effects and government contribution was negative for the first time in a while.
Importing into inventories counts as both investment and import, resulting in no net change to GDP. Consumers importing for themselves counts as both import and consumption, resulting in no net change to GDP
Noah Smith is a galaxy-scale idiot, by the way.