Dental record identifications are made from the whole dental picture normally. Which teeth are missing, which are filled-what surfaces, what material, have they had root fillings, are they chipped or worn, have any teeth had crowns etc.
The crucial factor is how big the fragment is, and what part of the tooth it came from if it is to be of any use for visual identification. A root from a molar tooth would be almost useless, (unless it was unusual-shape or size-and there were good pre existing x rays. Half of the clinical crown of a central incisor would be great.
It is reasonably easy to identify a human tooth visual, if there are any anatomical markers present. (Again it comes down to how big the fragment is, and fragment of what part of the tooth) I have read some posters speculate that it wouldn't be possible to determine that it is a human tooth. I don't agree with this at all. I think most I could identify, and I'm a dentist, not a forensic odontologist. (They are really clever!)
The fact that the fragment survived the fire is interesting. The rest of the teeth apparently didn't, and I wonder if this tooth wasn't somehow dropped or something before the fire took hold. (In my dreams I hope that it is the tooth that KL retrieved. And it may well be so)
I'm only speculating, but the DNA testing was probably of the pulp- the soft tissue within the tooth, which makes me think/hope that the tooth fragment wasn't too fire damaged.