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 shall perform
the penance (prescribed) for the murder of a Sudra.
In general, the penances for killing men from the four Varnas are mentioned below:
Manu 11.126. One fourth (of the penance) for the murder of a Brahmana
is prescribed (as expiation) for (intentionally) killing a Kshatriya,
one-eighth for killing a Vaisya; know that it is one-sixteenth for
killing a virtuous Sudra
So, we can't conclude from these verses that Shudras are considered by the Manu Smriti as equal to animals.