That implies you can use zombies as undead shields, but you need 4-6 zombies in a line to reliably stop all the fragments.
An 11 by 11 square of zombies with an empty space in the middle contains 120 zombies, and each of the 40 zombies the outermost rank has 4 zombies between them and the space in the middle. If a grenade goes off in that space, the first rank of zombies is hit by 100% of the available fragments, the second rank by 50%, the third rank by 25%, the fourth rank by ~12%, and the last rank by 6% - and that’s still lethal to around 30%-50% of them if a single grenade can kill 100 zombies in this perfect example.
Survivors are often tougher than zombies, and better armored. It’s still probably safest to throw grenades far enough away that the survivor isn’t in the blast radius.