Well, perhaps we disagree on this, but I do not equate fiction writing with lying, and I did not read his statement as you did anyway. And the fact that he is a talented writer makes his agenda-driven "art" only the worse...
Well, a plain reading of the discussion tells me they were discussing the difference between fiction and a so-called "non-fiction memoire" from Frey. Frey is under attack (and rightfully so) because his story was marketed as non-fiction. Doctorow and Kushner both write fiction, even if historical.
So in that sense, Frey lied, but Doctorow doesn't.