The way I understand this there was likely a very small population of elephants already without tusks, maybe some kind of genetic defect that happened to be an advantage once you add greedy people into the mix. Or what else is possible is that there was one off mutation that produced small family of elephants that poachers wouldn't touch and these spread over decades.
Think about this, if today Covid mutated to kill almost all people but spared 0.1% with a certain gene, you would suddenly observe entire human population becoming immune to Covid.
This does not necessarily mean any new genetic material or "nature responding". Maybe nobody noticed the gene and it suddenly became very visible and comprising large part of population (while it declined dramatically as elephant population did). We may have missed elephants without tusks because they were just freak of nature but now they become proliferating because it becomes an advantage.
EDIT:
Selective pressure is part of evolution process, but it is not enough for evolution.
Evolution requires both mutation AND selection.
So, if the mutation was present BEFORE poachers happened, you can't really say that Mother Nature RESPONDED to poaching by evolving new mechanism. The word "response" implies causal relationship between poachers and mutation+selection of better equipped elephants. But if the mutation has already been present then there is no causal relationship and this wasn't a new mechanism.