It sounds like it? But it seems weird that in January 2022, they would first look to 2021 when the tax return has not been filed (it then would revert to them looking at the 2020 return).  Maybe they will reassess things after the tax return is filed???

In regards to the Advance payment:

‘‘(2) REFERENCE TAXABLE YEAR.—Except as provided in paragraph (3)(A), the term ‘reference taxable year’ means, with respect to any taxpayer for any calendar year, the taxpayer’s taxable year beginning in the preceding calendar year or, in the case of taxpayer who did not file a return of tax for such taxable year, the taxpayer’s taxable year beginning in the second preceding calendar year.