Either end of fiscal year or new session of Congress.
He had to keep govt funded because the 2020 fiscal year ended September 2020. A CR was passed through the end of year.
The new Congress will appropriate through September 2022. A new Congress has the prerogative to appropriate its session as it deems fit.
Federal workforce is paid this month for time in November. Funds today pay them in January for December. Won’t need another CR but if necessary it can be done as late as late February.
They tried to trap POTUS with need to keep govt open mashed with pork and COVID stimulus.
So it appears he dispatched a team to brainstorm a way to counter and they came up with the 1974 Impoundment Control Act.
The letter you posted appears to have been used before this current round. I could be wrong but when the CCP controlled media cycles out, the public will see POTUS came through. Figure by end of week.
He issued this letter after his statement about signing the bill.
He issued another last night as well: