Zimbabwe is a third world country and people have been using gambling as mean to become rich. But, If you practically think about it only a few win from gambling not all are lucky. This tax might be a way to stop people from getting poor through gambling and it's addiction. I don't agree with your point of view on government getting rich through 10 percent of winning amount. As, I said earlier only a few will win but a majority will lose.
If the implemented tax doesn’t reduce the number of gamblers or their activities, the government will gain more income. But if it ends up discouraging people from gambling, then their revenue will decrease, and I’m sure they’ve done some studies before deciding to implement it.
As you mentioned, it’s a third-world country, and in such places, people are often desperate to make money even if it’s through gambling, which many see as a risky or “wrong” way.