Nicholas,
I too am interested in knowing your source.

The full text Abstract shows HEK293 A549-hACE2-TMPRSS2 cells from Germany were used in the study [1]. There are many suppliers of human embryonic kidney 293 (HEK-293)-derived cell lines, specifically designed for COVID-19 studies. For example, InvivoGen's ACE2-(TMPRSS2) cells express high levels of the host receptor ACE2 alone, or in combination with the host protease TMPRSS2, at their cell surface. Thus, these cells are permissive to infection by SARS-CoV-2 and Spike-pseudotyped lentiviral particles. Of note, the addition of TMPRSS2 in A549 cells only significantly increases their infectivity [in-house data]. This cell line collection is ideal for screening small molecule inhibitors and neutralizing antibodies that aim to block the virus-host interaction and cellular signaling outcomes [2].

293T cells are derived from the human embryonic kidney 293 cell line. 293T cells contain the simian virus 40 (SV40) large T-antigen, which facilitates replication of transfected plasmids containing the SV40 origin of replication. 293T/ACE2 cells are transfected to express ACE2, AND HAVE BEEN USED FOR MANY SARS-COV CELL-CELL FUSION AND PSEUDOVIRUS ENTRY INHIBITOR STUDIES [3]. All caps for emphasis. That 293T has been used for many studies does not mean they are the only cells that can or should be used.

Source 1:  www.biorxiv.org/.../2021.03.19.435959v1.full
Source 2:  https://www.invivogen.com/covid-19-related-cells
Source3 :  https://covdb.stanford.edu/cells-list/