If I'm compelled to think about it I think of "us" as an abbreviation for "compared to us", and if anyone wants to tell me that's wrong, tell about 60 million other people too.
"Us" is and was correct, and whatever the misguided pedant below says it's sad when so much important has not been corrected that this has been altered.
Pop60, congratulations for your beautiful 200 days streak ! I am sure that the translation "Some are as smart as us." is accepted. Every sentence has many accepted translations, and new translations are still added from users reports, but it is hard to choose a "best one" and to fit everyone taste. I feel that you think that "Some are as smart as we are" is not the better choice...
Sad that this has not been corrected and that the minority views of one unfortunately misguided contributor have apparently influenced Duolingo.
"She's a better dancer than me" is as acceptable as "She's a better dancer than I am", and if the final verb is omitted then the use of "me" becomes virtually mandatory. "She's a better dancer than I" spoken by an English person in England would give what might loosely be described as a snobbish and unpleasant impression. There may be historic rules - some of us were brought up with them, but I think we're dealing with 21st century English here, not 19th century.