Did we actually hoard the supply?
The way I heard it, contractual arrangements with the vaccine producers forbid the US government from providing the emergency-use-authorization vaccines outside the US, as the manufacturers required the liability shield that comes with the emergency-use regulations. I also understand that if another country offers to establish similar liability protections, that contract requirement can be lifted. As happened with Mexico and Canada, and thus our "loan" of doses.
We also speculatively purchased lots of doses of the various types, before even knowing if even one would pan out, who knew all the bets would pay off?
It seems a more complicated situation than "Scrooge McDuck is hoarding all the doses under his bed". And as usual with such things, lawyers are involved :-)