Ex-Dell-HR here who worked with the team who ran TellDell. TD is confidential, not anonymous.
Anonymous = impossible to know the respondent.
Confidential = steps taken to protect respondents' identities.
TellDell is outsourced to Perceptyx. They administer the survey and handle all the reporting back to leaders. They have direct access to who responded, so it is therefore not anonymous. However, there is a hard wall between Perceptyx and Dell, making it confidential. They do not share any respondent information with the Dell HR team managing the relationship, there is no dashboard on the Dell side that provides respondent-level access, and the answer from Perceptyx would be a professional "go to he-l" even if Dell requested it.