They do not need to. You are missing the point. Assuming the inscription is genuine, then it strengthens the notion that these personalities mentioned in the sources existed.
But not as Companions who lived in Mecca/Medina/Zem-Zem. These personalities do not attests themselves that they are "Companions" of any "prophet" and that they come from "Mecca/Medina/Zem-Zem". In any contemporary documents.
The content of the inscription does support this: mentions the Messenger and the three persons who witnessed.
Mentions the Messenger that is described in the Quranic texts, nothing else.
The content of the inscription, the persons, match with what the sources tell us as well, the dates, and the location, that is. Anyways.
But these personalities never mentions (in any documents) that they know the Messenger, that they are "Companions" of any "prophet" and that they come from "Mecca/Medina/Zem-Zem"
Therefore nothing in these inscription attests what say the narrative about them.
As I already said, the narrative of the 9th c. attributes to personalities of the past a history that these personalities never
corroborate or validate, by allusions in any document (epigraphic, archaeologic or scribal). Ne-ver.
Therefore these inscription attest of nothing about the origin of the Quranic text, and do not attest (at all) the existence of a "prophet". The only reason they mention a "messenger" is the Quranic texts.