What are you driving at bringing up the semantics of ‘contemporary’??

The only time that word was used was when you said (incorrectly), “That is contemporary literary evidence of his existence.” – the annals are centuries after the 6th-century reign of Diarmait at Tara. We don’t have any 6th-century manuscripts. The situation in the Roman Empire is quite a bit better, lots of texts.

Would you say that a person called Caius Vipstanus existed because Tacitus mentioned him in his annals a few decades later? Isn’t that valid inference from the text?