It appears you may be correct:
P.6 of the Regina Coeli report in which the picture appeared mentions that these accounting women are stationed at a building nearby called the “Infant Jesus House.”
Therefore, they are definitely employees of the SSPX, and working on the premises of SSPX property.
This article provides some information about the Infant Jesus House:https://sspx.org/en/locations/district-house
In that case, it would definitely be within the authority of the SSPX to insist upon a dress code
As to whether or not these women are SSPX parishioners or not, there are some comments in the Regina Coeli report which featured the picture which suggest such to be the case, but which are nevertheless inconclusive.
In any case, I hope my readiness to modify the thread if need be evinces the good spirit and disposition which motivated it.