Doing a bit more digging on this. You can download additional CSV files by altering the week number in the file name (i.e. NCHSData11.csv, NCHSData12.csv, NCHSData13.csv, etc.). You will notice that the data for a given week in 2020 increases as time goes on.
Reviewing the methodology, CDC states that they publish data for the previous weeks (including last week) from death certificate data. They further state that they update previous weeks as more localities release their certificate data. So, 2020 data is not yet complete.
https://www.cdc.gov/flu/weekly/#s2
https://www.cdc.gov/flu/weekly/overview.htm#anchor_1539281356004