If you had hl2 or episode 2 without steam, then you pirated it. Simple as that. Both games were never released without steam (though hl2 was leaked.)You will NEVER get the mod to work through the normal methods if you use an illegal copy like that. Go buy the Orange box, it is well worth the money.
I guess he wanted to say that he had Half-Life 1 back in the times of WON and bought Episode 2 as retail, but for some reason, he didn't register it with Steam at first. However, Empires doesn't need anything to show up in the My Games-list. Steam doesn't need a reinstall if the registry is fucked.
You misunderstood what he said, he said you don't need anything else installed to have Empires show up, and he's right.
But you do need to own a source engine game. And by own I mean you either paid for it or it was gifted to you. Free HL2:DM offers don't count.
No, it shows up even if you don't have a source game. But you need a source game in order to play Empires. Example: <User> just installed Steam and created an account. There aren't any source games registered on it. <User> now installs Empires and restarts Steam. Empires shows up and he starts it. But it tells him that he doesn't own a source game and that's why can't play Empires. After that, <User> buys Half-Life 2, then he starts Empires again and is able to play it.