What a silly card. Okay, so Angrath is attacking with a Steadfast Armasaur. Nahiri blocks with a Bear Cub. Angrath activates Armasaur, targeting Bear Cub. In response, Nahiri casts Unsummon, targeting Armasaur.

According to the Gatherer rulings (which as we all should know by now aren't 1000% correct all the time), it uses last known information to deal butt-damage to the Cub. (FWIW, it works this way on MTGO as well.) But .. shouldn't this this ability be countered? The Bear Cub is still blocking (per 509.1h), but it's not blocking Armasaur, since it's no longer on the battlefield. 608.2b seems to indicate that it might use last known information to inform target legality. But then there's this scenario:

Angrath controls a Bear Cub, Nahiri controls a Siren Stormtamer. Angrath casts Skullduggery, giving Bear Cub +1/+1 and Stormtamer -1/-1. Nahiri activates Stormtamer's ability. Skullduggery doesn't get countered. So why doesn't Stormtamer use LKI to determine target legality, but Armasaur does?

Or is something goofed up?