There is this following verse (11.140) which supports the commentator's interpretation (mentioned in this answer), that when a number of animals have been killed, one must perform the penance of killing a human being viz: a Shudra.
Manu 11.140. But for destroying one thousand (small) animals that have
bones, or a whole cart-load of boneless (animals), he ...
Medhatithi in his commentary for this verse says:
Inasmuch as the expiation prescribed is a heavy one, it should be understood as applying to a case where all these animals have been killed.
The purpose of this verse is to enjoin the type of penance that must be done for killing all these animals together.
So, the verse is saying that the penance (...