The study that you are referring to took place "Between July 27, 2020, and November 14, 2020". The delta variant started in India in late 2020 and wasn't spread

The study that you are referring to took place "Between July 27, 2020, and November 14, 2020". The delta variant started in India in late 2020 and wasn't spreading widely within the United States until 2021. It was listed as a variant-of-concern by the WHO on May 31, 2021. So the research you are referring to is in regards to other variants of sars-cov2