I've previously written about this here. To quote the relevant section on hoarding from V Was for Victory:

"A study of fourteen cities showed hoarding under way in many items: food (especially sugar, canned meats and vegetables, coffee, tea, spices, and olive oil), rubber goods (used automobile tires, gaskets for jars, garden hoses, golf balls, galoshes, girdles), household supplies (soap, linen, furniture, blankets), clothing (particularly men’s suits and shoes), and a miscellany of rifles and shotgun shells, typewriters, and paper clips."

I've not run across the comment about food shipments from overseas counting against a ration book before and a very quick look through a few sources didn't turn up anything. It's not impossible that may have been the case - at least theoretically - but what also came to mind is just how difficult it would have been to get imports on a consumer retail level for most of the war, especially since civilian travel overseas during the war's duration was a rarity too unless it was already considered vital to the war effort so unless you're talking about returning troops I'm not really sure there was all that much opportunity.

Also, on a wider level, the massive drop in available supply of internationally sourced goods like oils and sugar (Japan's occupation of various parts of the Pacific had essentially eliminated palm oil, and sugar imports kept getting sunk off the Atlantic coast by U Boats through 1944) were some of the main reasons why rationing was implemented in the first place. My hunch would be that what you're referring to might have been a policy related to someone bringing something back into the United States that was currently rationed, but in my experience there's a lot of mythology out there about the rationing program and this may be part of it.