That article never claims that those 6 episodes were Leslie's only appearances. In addition to those episodes, Leslie definitely appeared in The Ultimate Computer, Assignment: Earth, And The Children Shall Lead and The Enterprise Incident, since he was called by name.
Fair enough. However, since "Obsession" would've been an important show in the character's life, the fact that it's not is very telling that that's not Leslie. And on top of that, beyond the actor (which is meaningless, since actors play a lot of different characters), there's no evidence that Leslie was the "Obsession" redshirt and a lot of evidence that he wasn't (since Leslie was alive after the fact).