During Joe Biden’s presidency, Democrats held effective control of both the House and the Senate in the 117th Congress from January 20, 2021, until January 3, 2023, giving them a governing trifecta. (en.wikipedia.org)

In that period, Congress passed and Biden signed into law all of the major Democratic-led legislative packages that managed to clear both chambers, including:

• The American Rescue Plan Act of 2021 (COVID relief), passed on party-line votes and signed March 11, 2021. (en.wikipedia.org)
• The Infrastructure Investment and Jobs Act (“bipartisan infrastructure law”), enacted in 2021. (en.wikipedia.org)
• The CHIPS and Science Act, signed August 9, 2022. (bidenwhitehouse.archives.gov)
• The Inflation Reduction Act of 2022, the core climate/health and tax package. (en.wikipedia.org)

Official veto records from the U.S. Senate and compiled veto lists show that Biden issued no vetoes at all during the 117th Congress; his vetoes begin only in 2023, after Republicans captured the House in the 118th Congress. (senate.gov) Those later vetoes were almost entirely aimed at Republican-led congressional disapproval resolutions targeting his administration’s regulations, not at blocking a Democratic Congress’s legislative agenda. (en.wikipedia.org)

Some ambitious progressive priorities (for example, the larger original Build Back Better framework and certain voting-rights or filibuster-reform efforts) failed because they could not secure unanimous Democratic support in the Senate, particularly from moderate Democrats; such proposals never passed both chambers, so Biden was never in a position to veto them. (en.wikipedia.org) These are therefore not counterexamples to the claim that he would sign what a Democratic Congress actually passes.

Given that (1) Democrats did control both chambers for the first two years of Biden’s term, (2) Biden signed the major legislation that Democratic leaders were able to move through Congress, and (3) he did not make a practice of vetoing or otherwise blocking bills passed by that Democratic Congress, Sacks’s prediction—that Biden would essentially "sign whatever [Democrats] pass" if they won Congress—has been borne out by subsequent events.