Dear Daniel,
HR department must be as old as history of employment. Duties might have changed in due course of time but responsibilities and the core reason for HR department still remains the same. With the time, use of technologies and tools must have raised the bar of engagement or evaluation of employees.
If any Engineer could have handled HR profile why do any company look for HR as specialized profile. Will engineer be able to fulfill the compliance or applicable laws under various acts and notifications, can he ensure safety of employees, can he analyse employee training needs, can he appraise an employee on his performance, can he define a growth chart for employee, can he solve employee grievances raised due to salaries or inter employee conflict, can an engineer organise recruitment process, ... If answers to all these questions are YES than he is actually an HR person what is he doing in Engineering. ?
And if your interviewer was wanting to make you think out of the box, then after defining the responsibilities of a HR you should have asked to be more specific on a particular job or task and then could have given your reply.
If you interview a driver what do you think he should be capable of doing other then driving and taking care of his vehicle maintenance? Out of box thinking would be to see if he can handle medical emergency like CPR or artificial respiration in case he bangs a vehicle
Either the interviewer was a naive or himself had no idea what he is expecting as a answer.