They subsequently auctioned off permits to hunt elephants. https://www.bbc.com/news/world-africa-51413420
There are other things which point to something other than poaching.
"It is only elephants that are dying and nothing else," Dr McCann said. "If it was cyanide used by poachers, you would expect to see other deaths." Dr McCann has also tentatively ruled out natural anthrax poisoning, which killed at least 100 elephants in Botswana last year.
But they have been unable to rule out either poisoning or disease. The way the animals appear to be dying - many dropping on their faces - and sightings of other elephants walking in circles points to something potentially attacking their neurological systems, Dr McCann said.
- TFA
I would imagine that hunters (who have already been issued permits to kill 60 elephants so far, see https://www.reuters.com/article/us-botswana-elephants/botswa... ) would take some part of the elephant as a trophy, so it probably isn't a result of the legal hunting permits.
Just to be pedantic and a bit conspiratorial, poaching is defined as the illegal hunting of an animal. Since lifting a ban on hunting elephants is definitely controversial, I would imagine it's in Botswana's government's interests to be specific in their wording. They've only "ruled out poaching as a reason," they haven't ruled out that these elephants were killed by people.
If the problem was permitted hunting... it wouldn't be a mystery.