In the current workflow of the new assignment enhancements feature, students cannot submit their submission from the 'comments' tab. Some students after attaching their work, will add a comment for the attention of their tutor. In this scenario, there is no 'submit' button in the 'Comment' tab. This may confuse students, thinking they may have submitted work, when they need to go back to the 'attempt 1' tab to hit the submit button.

 

See here for assignment enhancements page:

Student Assignment Enhancements Resources