"My aunt is very smart, she is a teacher."
if you click on family lessons you get at the bottom a small summary that for pronouns you don't need to use 'de'. Normally I would have used 'de' but because of that summary I skipped it and bam 'error'
"我阿姨很聪明，她是老师。" was marked wrong. So are we trying to say the aunt isn't a close enough family member to warrant dropping the 的
Is that how we differentiate when we can/cannot drop the de? If so that's quite a little tip!
No, both with and without 的 are correct (with 的 is slightly more formal). This is in beta and it takes some time to make all the corrections.