But how do we know the memory cards being sold elsewhere didn't originate from China?
Yeah, don't misunderstand what's being talked about here. The question isn't buying Chinese vs. buying American or something. It's buying Chinese products that have been imported officially into America by some company, gone through the whole regulatory process and had the requisite markup applied vs. just buying stuff *direct* from China with no "middleman" (or the US government) involved.
When Sandisk or something makes a memory card, they go to some Chinese manufacturer and sign a contract saying "make 10 million micro SD cards in 64GB capacity for such and such price" and the Chinese company then makes them to that specification, because they want to get another order from Sandisk for 10 million memory cards. Meanwhile, as an American company, Sandisk doesn't want to get sued in US court or fined by the FTC, so they sell memory cards at the rated capacity.
But some guy in China could just as easily contract with that same factory for, say, 1,000 2GB cards for $1 per piece, do whatever it is they do to make memory cards appear larger than they are in Windows, put his own labels on them and sell them on Ebay as 32GB cards for $3 each, and nobody's going to do anything to him. The Chinese government won't because they don't even know and probably wouldn't care. The US government won't because it's some guy in China and they have no jurisdiction. Ebay may or may not if enough people complain, but even if they did, by then he'd probably have sold a million of these.
So it's really about unscrupulous random people who could be anywhere, but in this case are almost always from China because that's where a lot of today's manufacturing is.
I do think you have to be really careful buying computer or electronic stuff direct from China. Some of it's fine and some of it isn't.