According to news reports, Grenfell survivors will have a say at the inquiry.
I think it would be asking rather a lot for people who have had this terrible experience, and who have possibly lost family and friends, to be wholly "objective" in the sense of being able to distance themselves from their emotions. They are bound to be emotional and, at times, distressed or angry.
I don't suppose people who have been sexually abused are able to be "objective" either but when there have been inquiries their testimonies have been a crucial part of the proceedings, and that's as it should be. Those who have suffered should be able to recount what happened to them and how it has affected their lives.
No doubt there will be other people who will be called who could also be claimed to not be wholly "objective" because they may be partly responsible for the tragedy. So it works both ways.