Its pretty funny US government uses Coinbase like that or maybe its deliberate kind of free advertising and inadvertently backing the idea of using Coinbase rather then more foreign exchanges possible to use or other avenues.    I always liked the ancient escrow site where BTC is exchanged but thats gone now sadly, theres enough newly invented paperwork to fell the rainforests for good at this point I think thats at least half the reason in many operations choosing to close down nowadays.

In terms of the amount sent, I believe they will not sell all at once or even within 1 month most likely it will distributed over time.   Same policy of a program to sell over time and ensure even liquidity as any large issuance policy is performed by.  US government being also the largest debtor to ever exist will surely be well versed in the idea of communicating with buyers to not overwhelm demand so I think its over a long time not a lump sum falling in and making a giant splash in the market.