The doc makes two distinct points which I exaggerate a little to make the distinction: 1) Was the assassination of Thompson justified? 2) Did it do any good?
I'll imply my answer with a parallel case. Was the assassination of Heydrich justified? Yes. 2) Did it do any good? Hmm, historians debate that! To make a thick answer, let's say no. 2a) Will the assassination of Thompson do any good? The doc says no. I am not so sure that it won't. But, to keep thickness, let's say no again. Does the answer to question two affect the answer to question one? No. Those are distinct points. The assassination of Heydrich stands unassailed as justified regardless of the historians' debate on whether any good came of it to the Czech people.