It happened over quite a long time period though. The couple I heard on the radio that called their family initially called them just to tell them about the fire, that they had been told to stay put, they called back later to say they couldn't get out saying their goodbyes.
For someone to totally disappear they would have to be
1. An illegal tennant
2. Have no friends or family they tried to reach during the fire
3. Didn't reach out to the emergency services at any time during the fire
4. Didn't have any social media that they posted on during the fire
5. Subsequently didn't have any friends or family that may have been distant or estranged looking for them afterwards
I think the number of people fitting that criteria would be pretty small.