Good luck getting a grenade through a small(few foot) hole while being shot at. It's not like they could run up to that hole without being shot multiple times. As for reach everyone, armored hands with guns can fit through it just fine if some of them dodge out of the area that was reachable. The way it was set up, almost all of the imperials would have started in the field of fire. Plus an armed Din was inside encased fully in beskar. He'd have been able to fight as well, taking out the grenade attempts. The imperials would have been toast if she had cut a hole there.
Edit: Plus, before they could even recover to the point of formulating the grenade strategy, Bo could have moved left and right, cutting new holes for even more mandalorians to shoot through, covering an even wider area with blaster fire at the imperials.