The paper[1] goes into this in more depth. COVID was definitely a factor, but not the only one. For example if you look at the YOY comparisons, both halves of 2021 hadlower emissions YOY than 2020. The H1 decrease you can blame on the COVID lockdowns that lasted until May. But by July 2020 everyone who was going to WFH was WFH, and by July 2021 many people who were WFH in 2020 were commuting again.

[1]: https://pubs.acs.org/doi/10.1021/acs.est.3c09642