Rerolling 3 champion shards when you own every champion already gives you a random champion permanent, which itself would be useless except for champion mastery (and just keeping the shards is probably wiser).
When a new champ is released, within a day or two (but not immediately), rerolling 3 champ shards would get you the new champ. (Although this tends to be pointless since you likely have plenty of spare ip by that point.)
You can't get hextech annie via reroll champ shards. They probably got confused as to what happens if someone owns all regular skins and rerolls 3 skin shards, as in that case you'd get hextech annie (it's always possible albeit rare, but if theres no other options, then hextech annie would be what you get). Of course, owning all skins isn't an issue most people have.