Currently, because the hosted feature layer associated with a public Survey 123 survey is shared to the public and allows adding new features, a public user could, if they were so inclined, search for and find the feature layer in ArcGIS Online, and add features to that layer through the Map Viewer in AGO, and not using the Survey 123 web form or app. The downfall of this is that someone could potentially add dozens of meaningless features to the survey's feature layer.
It would be ideal there were a way to restrict the adding of new records to the underlying feature layer of a public Survey 123 survey at the feature layer level based on what application is being used. Whether this would be a "referrer" setting in the survey to only allow new submissions from survey123.arcgis.com, or perhaps another setting in the Editing settings of the hosted feature layer itself, "only allow new records added using Survey 123 webform or app", or something to that effect.