Here is Christopher Hitchens defending Irving in 1996:
https://www.vanityfair.com/news/1996/06/hitlers-ghost-christopher-hitchens
He states he is a fascist historian, but also a great historian of fascism. Because of his sympathies with Nazi Germany, Irving actually got access to surviving Nazis for his books that did not speak to other historians.
The Mare's Nest managed to find German sources and research on the V1 and V2 rocket programmes which had eluded other scholars, but Irving still went out of his way to ignore or explain away the slave labour which built the programme.
There is an academic argument that would state that Irving managed to provide new perspectives on Nazi Germany, but to be of historical use you have to selectively read and interpret his work to get around the disturbingly apologetic tone he adopts for Nazi Germany. And that's before the later more overtime Holocaust denial.
Of course, for people without academic or historical training his books can be read as pure propaganda, and if you are in Finkelstein's position as an academic, a throwaway statement praising Irving's historical nous can be very misleading, to say the least.