First, Yates says that OLC did not take into account “statements made by an administration or its surrogates close in time to the issuance of an Executive Order that may bear on the order’s purpose.” I assume Yates is referring here to statements such as the one by Rudy Giuliani, who recently claimed that Trump wanted a “Muslim ban” and sought “the right way to do it legally. I am sure OLC didn’t take such statements into account, since they would not be relevant to review for form and legality. I can imagine these and similar statements properly informing the Attorney General’s view of the legality of the EO, if she believed that these statements amounted to the EO being motivated by invidious discrimination (though even if she concluded that, the relevance of such discrimination in the context of the immigration issues here is tricky). But Yates does not say she has concluded that, and it is pretty clear from the context of her letter (see below) that she has not ruled out that there are reasonable arguments in support of the EO.
Second, she says that OLC did not “address whether any policy choice embodied in an Executive Order is wise or just.” True, that is not OLC’s job. But nor is it the Attorney General’s—at least not if the President has decided that the policy choice is wise and just. The Attorney General can personally advise the President about an EO’s wisdom and justness. And the Attorney General can decide to resign if she thinks the President is pursuing a policy so unwise and unjust as to be morally indefensible. But an Attorney General does not typically (I cannot think of a counterexample offhand) refuse to defend an Executive Order in court because she disagrees with the policy basis for the EO.