The US government purchased those. They own those doses and so they are not AstraZenca's to sell.
From your own nytimes link:
>"Last May, the Trump administration pledged up to $1.2 billion to AstraZeneca to finance the development and manufacturing of its vaccine, which it developed with the University of Oxford, and to supply the United States with 300 million doses if it proved effective."
AstraZenca is free to export any vaccine beyond their contractual obligation of those doses. Jane Psaki confirmed as much in her WH press conference on Thursday.