This  argument  is  by  no  means  new  at  all.  It  was  around  in  the  days  when  Adam  Clarke  wrote  his  Bible  Commentary  a  few  hundred  years  back.  Here  is  what  he  has  to  say  on  this  argument  that agnostics  and  atheists  try  to  use  to  debunk  the  Bible.

But by my name Jehovah was I not known to them.  This passage has been a sort of crux criticorum, and has been variously explained. It is certain that the name Jehovah was in use long before the days of Abraham, see Gen. ii. 4, where the words Jehovah Elohim occur,  as they do frequently afterwards; and see Gen. xv. 2, where Abraham expressly addresses him by the name Adonai Jehovah; and see the 7th verse, where God reveals himself to Abraham by this very name: "And he said unto him, I am Jehovah, that brought thee out of Ur of the Chaldees." 

How then can it be said that by his name Jehovah he was not "known unto them"?    

Several answers have been given to this question;  the following are the chief: —

1. The words should be read interrogatively, for the negative particle, not, has this power often in Hebrew.  "I appeared unto Abraham, Isaac, and Jacob by the name of God Almighty, and by my name Jehovah was I not also made known unto  them?"  

2. The name Jehovah was not revealed before the time mentioned here, for though it occurs so frequently in the book of Genesis, as that book was written long after the name had come into common use, as a principal characteristic of God, Moses employs it in his history because of this circumstance; so that whenever it appears previously to this, it is by the figure called prolepsis or anticipation.     

3.  As   the  name  Jehovah signifies existence, it may be understood in the text in question thus:  "I appeared unto Abraham, Isaac,  and Jacob by my name God Almighty,  or God ALL-sufficient, i. e., having all power to do all good; in this character I made a covenant with them, supported by great and glorious promises;  but as those promises had respect unto their posterity, they could not be fulfilled to those fathers: but now, as Jehovah, I am about to give existence   to all those promises relative to your support, deliverance from bondage, and your consequent, settlement in the promised land."    

4. The words may be considered as used comparatively: though God did appear to those patriarchs as Jehovah, and they acknowledged him by this name, yet it "was but comparatively known unto them; they knew nothing of the power and goodness of God, in comparison of what the Israelites were now about to experience.

I believe the simple meaning is this, that though from the beginning the name Jehovah was known as one of the names of the Supreme Being, yet what it really implied they did not know. El-Shaddai, God All-sufficient, they knew well by the continual provision he made for them, and the constant protection he afforded them: but the name Jehovah, is particularly to be referred to the accomplishment of promises already made; to the giving them a being, and thus bringing them into existence, which could not have been done in the order of his providence sooner than here specified: this name therefore in its power and significancy was not known unto them; not fully known unto their descendants till the deliverance from Egypt and the settlement in the promised land. It is surely possible for a man to bear the name of a certain office or dignit, before he fulfils any of its functions. King, mayor, alderman, magistrate, constable, may be borne by the several persons to whom it legally belongs, before any of the acts peculiar to those offices are performed. The king, at acknowledged as such on his coronation, is know to be such by his legislative acts; the civil magistrate, by his distribution of justice, an issuing warrants for the apprehending of culprits; and the constable, by executing those warrants. All these were known to have their respective names, but the exercise of the powers alone shows what is implied in being king, magistrate, and constable. 

The following is a case in point, which fell within my own knowledge.

A case of dispute between certain litigious neighbours being heard in court before a weekly sitting of the magistrates, a woman who came, as an evidence in behalf of her bad neighbour, finding the magistrates inclining to give judgment against her mischievous companion took her by the arm and said, "Come away. I told you, you would get neither law nor justice in this place." A magistrate, who was as much an honour to his function as he was to human nature, immediately said, "Here, constable take that woman and lodge her in Bridewell that she may know there is some law and justice in this place."

Thus the worthy magistrate proved he hid the power implied in the name by executing the duties of his office. And God who was known as Jehovah, the being who makes and gives effect to promises, was known to the descendants of the twelve tribes to be THAT JEHOVAH, by giving effect and by being to the promises which he had made to their fathers.