Whether to Take the Inscription From 1191 Seriously?

There are no surviving documents from earlier than the year one thousand that make any reference to arthur being a king. So referring to arthur as rex in this inscription seems highly dubious and very likely to be a later forgery. The other aspect of the inscription that really raises doubt in my mind is the ending, where it says, in the isle of avalon. Have you ever seen an epitaph or inscription that specifically makes note of where it is? It's totally redundant and useless in a real inscription. And there doesn't seem to be any reason to add it on there, unless the author is insistent on connecting that location to avalon and is trying to prove that