Seems is not for sure. Even given that, all that would tell you is that it existed around 1192. No evidence that it was any older.
historians have speculated...that what is now known as the Shroud of Turin was known to the Byzantines as the Holy Mandylion of Edessa that has a historical trail back to the 6th century but which disappeared (coincidentally) at the sack of Constantinople in 1204.
Historians speculated? Speculated as in "made an educated guess"? That doesn't constitute any proof at all. Spend any amount of time watching the History Channel and you'll quickly realize historians often speculate (often wrongly) all the time. Speculation isn't evidence in favor or against a proposition.
I am being extra careful with “seems” and “speculated” as I don’t want to stretch the evidence farther than it goes. You want more documentary evidence on the connection with the Mandylion and the Abgar image, here you go:
“King Abgar received a cloth on which one can see not only a face but the whole body” (in Latin: [non tantum] faciei figuram sed totius corporis figuram cernere poteris)—A seventh-century account from the Codex Vossianus Latinus
“The Venetians partitioned the treasure of gold, silver and ivory, while the French did the same with the relics of saints and the most sacred of all, the linen in which our Lord Jesus Christ was wrapped after His death and before the resurrection.” Letter from Theodore Ducas Anglelos to Pope Innocent III, 1205
And some more:
* 6th Century: “The divinely wrought image which the hands of men did not form.”
* 8th Century: “The Lord (…) impressed an image of himself.”
* 9th Century: “…the city of Edessa in which there was preserved a blood stained image of the Lord, not made by hands.”
* 10th Century: “A moist secretion without pigment or painter’s art.”
* 10th Century: “The splendor has been impressed uniquely by the drops of agony sweat (…) These are truly the beauties that produced the coloring of Christ’s imprint, which has been embellished further by the drops of blood sprinkled from his own side.”
* 12th Century: “Abgar reigned as Toparch of Edessa. To him the Lord Jesus sent (…) a most precious cloth, which he wiped the sweat from his face, and on which shone the savior’s features, miraculously reproduced. This displayed to those who gazed upon the likeness and proportions of the body of the Lord.”
* 13th Century: “The story is passed down from the archives of ancient authority that the Lord prostrated himself with his entire body on the whitest linen and so by divine power there was impressed on the linen a most beautiful imprint of not only the face, but the entire body of the Lord.”
I’d really recommend you read more on this. I have plenty of experience in historical research and my nose is often buried in primary source documents from the Roman Republic on. For my part, I think the connection is pretty solid. Your mileage may vary, but it’s certainly not a case of “no evidence”.