It's usually explained as "A small band of heroes" or "Famous NPC and a small group of heroes," for example, for Illidan in Black Temple, it would have been "Maiev Shadowsong and a group of heroes defeated Illidan Stormrage after storming through the temple and defeating Illidans minions."

In terms of YOUR personal perspective on it, you can put your own spin on it. You can choose to say "I was there, I helped kill Illidan, I was a part of the group of heroes." Or you can say, "Well, it wasn't me, but I was able to vicariously experience that event in an alternate reality universe that allowed me to re-enact those noble deeds." There is always room for interpretation of the "and a group of heroes," because Blizzard never specifies which heroes, and in fact rarely even hints at which faction did the deed.

That is the fun of an open-ended MMO, you can make of it whatever you wish. Blizzard will usually identify certain lore characters as leading the fight (Tirion, Varian, Maiev), and the rest is up to you to role play or simply decide what you really think happened.