Defence? Of course it is
"although Rajlich repeatedly denied plagiarizing other programmers’ work, he didn’t offer “any kind of defence to the allegations”."
Surely, "didn't do it" is a defence? Chess programs only do so many different things, and if he wrote a chess propram that just "happened" to be very similar to another chess program, even though he had never seen the "other" chess program, is that plagiarism? No. There is no other defense he can present except that he didn't copy the other program.
This not a comment on whether or not he did it, simply a comment on the fact that you can't prove a negative, "prove you didn't copy it" = "prove there is no god." Can't be done, surely it should be up to the accuser to prove that he DID do it!