None of the heroes specifically knew where Scott was when half of life in the universe was eliminated, so when he didn’t turn up after the event it was just assumed that he was dusted (the reality actually being that he was trapped in the Quantum Realm).
All that being said, there actually was good reason to believe when the first trailer said that Shuri was counted among the Decimated.
After all, her situation at the time of the Thanos snap was a lot different than Scott Lang’s.
Shuri, on the other hand, was present in Wakanda with the Avengers battling against the Black Order and their Outriders, so the team knew exactly where she should have been… and clearly she wasn’t there.