He says that the Diatessaron was a harmony of the Gospels, i.e.( in his sense) of our present Gospels, and that Tatian gave the name of Diatessaron to his work himself.

We do not know upon what these statements rest, but there ought to be some valid reason before we dismiss them entirely.
Epiphanius writes that 'Tatian is said to have composed the Diatessaron Gospel which some call the "Gospel according to the Hebrews"' [Endnote 240:1].

And Theodoret tells us that 'Tatian also composed the Gospel which is called the Diatessaron, cutting out the genealogies and all that shows the Lord to have been born of the seed of David according to the flesh.' 'This,' he adds, 'was used not only by his own party, but also by those who followed the teaching of the Apostles, as they had not perceived the mischievous design of the composition, but in their simplicity made use of the book on account of its conciseness.' Theodoret found more than two hundred copies in the churches of his diocese (Cyrrhus in Syria), which he removed and replaced with the works of the four Evangelists [Endnote 240:2].
Victor of Capua in the sixth century speaks of Tatian's work as a 'Diapente' rather than a 'Diatessaron' [Endnote 240:3].

If we are to believe the Syrian writer Bar-Salibi in the twelfth century, Ephrem Syrus commented on Tatian's Diatessaron, and it began with the opening words of St.John.This statement however is referred by Gregory Bar-Hebraeus not to the Harmony of Tatian, but to one by Ammonius made in the third century [Endnote 241:1].
Here there is clearly a good deal of confusion.
But now we come to the question, was Tatian's work really a Harmony of our four Gospels?
The strongest presumption that it was is derived from Irenaeus.

Irenaeus, it is well known, speaks of the four Gospels with absolute decision, as if it were a law of nature that their number must be four, neither more nor less [Endnote 241:2], and his four Gospels were certainly the same as our own.