Another idea. Celsus distinguishes between Jews and Christians because the former were well established:
As the Jews, then, became a peculiar people, and enacted laws in keeping with the customs of their country, and maintain them up to the present time, and observe a mode of worship which, whatever be its nature, is yet derived from their fathers, they act in these respects like other men, because each nation retains its ancestral customs, whatever they are, if they happen to be established among them. And such an arrangement appears to be advantageous, not only because it has occurred to the mind of other nations to decide some things differently, but also because it is a duty to protect what has been established for the public advantage; and also because, in all probability, the various quarters of the earth were from the beginning allotted to different superintending spirits, and were thus allotted among certain governing powers, and in this manner the administration of the world is carried on. And whatever is done among each nation in this way would be rightly done, wherever it was agreeable to the wishes of the superintending powers, while it would be an act of impiety to get rid of the institutions established from the beginning in the various places. We must observe the laws because it is a duty to protect what has been enacted; because of the superintendents distributed among the different parts of the earth, what is done among each nation is rightly done.
Celsus doesn't particularly like the Jews and refers to them in disparaging terms. But could those words be used of a tradition that was founded in 270 CE? I don't think so. If Celsus had run across a story of the Jews having written their text recently in Alexandria he couldn't have written the following either:
History proves the claims to great antiquity put forth by many nations, as the Athenians, and Egyptians, and Arcadians, and Phrygians, who assert that certain individuals have existed among them who sprang from the earth, and who each adduce proofs of these assertions. The Jews, then, leading a grovelling life in some corner of Palestine, and being a wholly uneducated people, who had not heard that these matters had been committed to verse long ago by Hesiod and innumerable other inspired men, wove together some most incredible and insipid stories, viz., that a certain man was formed by the hands of God, and had breathed into him the breath of life. [Genesis 2:7.] Thus fashioned by the hands of God, the man was inflated by breath blown into him. A woman was taken from his side, and God issued certain commands, and a serpent opposed these, and gained a victory over the commandments of God; the serpent counteracted the injunctions given by God to the man. Thus they relate certain old wives’ fables, and most impiously represent God as weak at the very beginning of things, and unable to convince and gain over to obedience even a single human being whom he himself had formed.
The more modest among Jews and Christians are ashamed of these things, and endeavor to give them somehow an allegorical signification. They speak, in the next place, of a deluge, and of a monstrous ark, having within it all things, and of a dove and a crow as messengers, falsifying and recklessly altering the story of Deucalion; not expecting, I suppose, that these things would come to light, but imagining that they were inventing stories merely for young children. Altogether absurd, and out of season is the account of the begetting of children, the conspiracies of the brothers, a father’s sorrow, the crafty procedure of mothers. God presented his sons with asses, and sheep, and camels! God gave wells also to the righteous. The story of Lot and his daughters is worse than the crimes of Thyestes. The narrative describes the hatred of one brother for another, brothers who treacherously sally out on account of the insult offered to their sister, who had been violated by the son of a king, brothers selling one another, a brother sold, and a father deceived. They tell of the dreams of the chief butler and chief baker, and of Pharaoh, and of the explanation of them, in consequence of which Joseph was taken out of prison in order to be entrusted by Pharaoh with the second place in Egypt.
He who had been sold behaved kindly to his brethren who had sold him, when they were suffering from hunger, and had been sent with their asses to purchase provisions. Joseph made himself known to his brethren; Joseph, who had been sold as a slave, was restored to liberty, and went up with a solemn procession to his father’s funeral. [Genesis 50:7.] By whom (Joseph, namely) the illustrious and divine nation of the Jews, after growing up in Egypt to be a multitude of people, was commanded to sojourn somewhere beyond the limits of the kingdom, and to pasture their flocks in districts of no repute.
If the proofs or evidence for the the Pentateuch having been "invented" in Alexandria by Jewish writers having access to the library of Alexandria and the Greek works contained therein, he would certainly have mentioned it in the True Word. If such information was beyond an educated man like Celsus the evidence to support Gmirkin's hypothesis likely didn't exist anywhere.