I partly agree with him.
I think a public inquiry would have been possible, even though it would've had to be conducted mostly in camera. There are precedents for these kinds of inquiries, however. For example, the Public Order Emergency Commission chaired by Judge Paul Rouleau was partly held that way and the Arar Commission extensively so.
In this case, perhaps it could have been conducted more in camera because the information was extremely sensitive, although findings could have been published after the fact.
In my opinion, the more relevant argument for not holding a public inquiry is that it wouldn't have added any substantial value, given all the processes under way, such as the proceedings of this committee and those of the National Security and Intelligence Committee of Parliamentarians and the National Security and Intelligence Review Agency.
My assessment is ultimately somewhat different from that of Mr. Johnston. I think it would've added limited value.