The mission was approved in 2015, which means it was designed before then, we've learned a good chunk about how smaller asteroids bind together since then, the

The mission was approved in 2015, which means it was designed before then, we've learned a good chunk about how smaller asteroids bind together since then, the hyabusa 2 and osiris rex missions were both after 2019, so it wasn't wholely unreasonable that some other kind of internal binding mechanism might be at play besides the piddling role gravity plays at these scales.