Bundy did nothing but lie on death row. His last lie was his last ditch appeal to postpone his execution so he could take investigators to the bodies of women he claimed to have murdered, but told no one about.
Simply put, Bundy lied to save his ass. Bundy lied to Dobson in the hopes of winning Dobson's sympathy, and hoping to leverage that sympathy into something that could save his ass.
Of course he was a liar; but why lie about what motivated him? Dobson could do nothing to save Bundy, and Bundy knew it. You either haven't read/seen the actual interview or at least don't remember it, because his description of how violent pornography motivated him rang true.