It's interesting that this isn't actually illegal to do except in the specific context of an exchange market. I did a very cursory search of the US Code and the Securities Exchange Act of 1934 and yeah, unless there's some additional legal precedent or other applicable law I didn't find, then this might just be a gap in the law.

It even seems to skirt around notions of illegal vertical integration. For example in this address from 1998, a former FTC commissioner describes several types of illegal "vertical alliances", all of which rely on both the upstream supplier and the downstream consumer being aligned in anticompetitive intent, which (if the article is to be believed) they couldn't have been here because there are two suppliers who were unaware of each other's deals.

Is it really not illegal to just buy up a huge chunk of a critical input for an industry and stockpile it for the purpose of locking out competitors? Seems hard to imagine that some robber baron of the 19th century didn't already do this.

No, they want DRAM to be expensive to give them a competitive advantage over their competitors.